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Obsolete - kept for historical reason. Use Discussion page if necessary.
See http://wiki.tldp.org/Partitions-and-mass-storage-HOWTO for recent info.
Discussion Page Content
This HOWTO is now maintained by jdd, with the bless of the previous maintainer. As the subject is now much more complex, we open a new HOWTO and wont change this one if not for typo fix.
Original HOWTO
Linux Partition HOWTO
Anthony Lissot
Revision History
Revision 3.5 26 Dec 2005
reorganized document page ordering. added page on setting up swap space.
added page of partition labels. updated max swap size values in section 4.
added instructions on making ext2/3 file systems. broken links identified by
Richard Calmbach are fixed. created an XML version.
Revision 3.4.4 08 March 2004
synchronized SGML version with HTML version. Updated lilo placement and swap
size discussion.
Revision 3.3 04 April 2003
synchronized SGML and HTML versions
Revision 3.3 10 July 2001
Corrected Section 6, calculation of cylinder numbers
Revision 3.2 1 September 2000
Dan Scott provides sgml conversion 2 Oct. 2000. Rewrote Introduction. Rewrote
discussion on device names in Logical Devices. Reorganized Partition Types.
Edited Partition Requirements. Added Recovering a deleted partition table.
Revision 3.1 12 June 2000
Corrected swap size limitation in Partition Requirements, updated various
links in Introduction, added submitted example in How to Partition with
fdisk, added file system discussion in Partition Requirements.
Revision 3.0 1 May 2000
First revision by Anthony Lissot based on Linux Partition HOWTO by Kristian
Koehntopp.
Revision 2.4 3 November 1997
Last revision by Kristian Koehntopp.
This Linux Mini-HOWTO teaches you how to plan and create partitions on IDE
and SCSI hard drives. It discusses partitioning terminology and considers
size and location issues. Use of the fdisk partitioning utility for creating
and recovering of partition tables is covered. The most recent version of
this document is [http://lissot.net/partition] here. The Turkish translation
is [http://belgeler.org/howto/partition-howto.html] here.
-----------------------------------------------------------------------------
Table of Contents
1. Introduction
1.1. What is a partition?
1.2. Other Partitioning Software:
1.3. Related HOWTOs
1.4. Additional information on your system:
2. Devices
2.1. Device names
2.2. Device numbers
3. Partition Types
3.1. Partition Types
3.2. Foreign Partition Types
3.3. Primary Partitions
3.4. Logical Partitions
3.5. Swap Partitions
4. Partitioning requirements
4.1. What Partitions do I need?
4.2. Discussion:
4.3. File Systems
4.4. Swap Partitions
5. Partitioning with fdisk
5.1. fdisk usage
5.2. Four primary partitions
5.3. Mixed primary and logical partitions
5.4. Submitted Examples
6. Labels
6.1. Volume Labels
6.2. Device Labels
7. Formatting an ext2/3 partition
.1. Simple Invocation
.2. Reserved blocks
8. Recovering a Deleted Partition Table
9. Setting Up Swap Space
9.1. Swap Files
9.2. Swap Files
9.3. Multiple Swap Areas
10. Appendix
10.1. Formating Partitions
10.2. Activating Swap Space
10.3. Mounting Partitions
10.4. Some facts about file systems and fragmentation
1. Introduction
1.1. What is a partition?
Partitioning is a means to divide a single hard drive into many logical
drives. A partition is a contiguous set of blocks on a drive that are treated
as an independant disk. A partition table (the creation of which is the topic
of this HOWTO) is an index that relates sections of the hard drive to
partitions.
Why have multiple partitions?
* Encapsulate your data. Since file system corruption is local to a
partition, you stand to lose only some of your data if an accident
occurs.
* Increase disk space efficiency. You can format partitions with varying
block sizes, depending on your usage. If your data is in a large number
of small files (less than 1k) and your partition uses 4k sized blocks,
you are wasting 3k for every file. In general, you waste on average one
half of a block for every file, so matching block size to the average
size of your files is important if you have many files.
* Limit data growth. Runaway processes or maniacal users can consume so
much disk space that the operating system no longer has room on the hard
drive for its bookkeeping operations. This will lead to disaster. By
segregating space, you ensure that things other than the operating system
die when allocated disk space is exhausted.
-----------------------------------------------------------------------------
1.2. Other Partitioning Software:
* sfdisk: a command-line version of fdisk
* cfdisk: a curses-based version of fdisk
* parted: Gnu partition editor
* Partition Magic: a commercial utility to create, resize, merge and
convert partitions, without destroying data.
* Disk Drake: a Perl/Gtk program to create, rsize, and delete partitions
-----------------------------------------------------------------------------
1.3. Related HOWTOs
Table 1. Related HOWTOs
+------------------------+----------------------------+-------------------------+
|Title |Author |Description |
+------------------------+----------------------------+-------------------------+
|[http://www.tldp.org/ |[mailto:tobbyb@usa.net] |How to estimate the |
|HOWTO/ |Gjoen Stein |various size and speed |
|Install-Strategies] Dual| |requirements for |
|boot install strategies | |different parts of the |
| | |filesystem. |
+------------------------+----------------------------+-------------------------+
|Linux Multiple Disk |[mailto:sgjoen@mail.nyx.net]|How to estimate the |
|System Tuning |Gjoen Stein |various size and speed |
| | |requirements for |
| | |different parts of the |
| | |filesystem. |
+------------------------+----------------------------+-------------------------+
|Linux Large Disk |Andries Brouwer |Instructions and |
| | |considerations regarding |
| | |disks with more than 1024|
| | |cylinders |
+------------------------+----------------------------+-------------------------+
|Linux Quota |Ralf van Dooren |Instructions on limiting |
| | |disk space usage per user|
| | |(quotas) |
+------------------------+----------------------------+-------------------------+
|Partition-Rescue |Jean-Daniel Dodin |How to restore linux |
|mini-HOWTO | |partitions after they |
| | |have been deleted by a |
| | |Windows install. Does not|
| | |appear to preserve data. |
+------------------------+----------------------------+-------------------------+
|Linux ADSM Backup |Thomas Koenig |Instructions on |
| | |integrating Linux into an|
| | |IBM ADSM backup |
| | |environment. |
+------------------------+----------------------------+-------------------------+
|Linux Backup with MSDOS |Christopher Neufeld |Information about MS-DOS |
| | |driven Linux backups. |
+------------------------+----------------------------+-------------------------+
|Linux HOWTO Index |[mailto: |Instructions on writing |
| |linux-howto@sunsite.unc.edu]|and submitting a HOWTO |
| |Tim Bynum |document |
+------------------------+----------------------------+-------------------------+
-----------------------------------------------------------------------------
1.4. Additional information on your system:
* /usr/src/linux/Documentation
+ ide.txt: Info about your IDE drivers
+ scsi.txt: Info about your SCSI drivers
-----------------------------------------------------------------------------
2. Devices
There is a special nomenclature that linux uses to refer to hard drive
partitions that must be understood in order to follow the discussion on the
following pages.
In Linux, partitions are represented by device files. These are phoney files
located in /dev. Here are a few entries:
brw-rw---- 1 root disk 3, 0 May 5 1998 hda
brw-rw---- 1 root disk 8, 0 May 5 1998 sda
crw------- 1 root tty 4, 64 May 5 1998 ttyS0
A device file is a file with type c ( for "character" devices, devices that
do not use the buffer cache) or b (for "block" devices, which go through the
buffer cache). In Linux, all disks are represented as block devices only.
-----------------------------------------------------------------------------
2.1. Device names
2.1.1. Naming Convention
By convention, IDE drives will be given device names /dev/hda to /dev/hdd. H
ard Drive A (/dev/hda) is the first drive and Hard Drive C (/dev/hdc) is the
third.
Table 2. IDE controller naming convention
+---------------+----------------+---------------+
| drive name |drive controller| drive number |
+---------------+----------------+---------------+
| /dev/hda | 1 | 1 |
+---------------+----------------+---------------+
| /dev/hdb | 1 | 2 |
+---------------+----------------+---------------+
| /dev/hdc | 2 | 1 |
+---------------+----------------+---------------+
| /dev/hdd | 2 | 2 |
+---------------+----------------+---------------+
A typical PC has two IDE controllers, each of which can have two drives
connected to it. For example, /dev/hda is the first drive (master) on the
first IDE controller and /dev/hdd is the second (slave) drive on the second
controller (the fourth IDE drive in the computer).
You can write to these devices directly (using cat or dd). However, since
these devices represent the entire disk, starting at the first block, you can
mistakenly overwrite the master boot record and the partition table, which
will render the drive unusable.
Table 3. partition names
+--------------+--------------+--------------+--------------+--------------+
| drive name | drive | drive number |partition type| partition |
| | controller | | | number |
+--------------+--------------+--------------+--------------+--------------+
| /dev/hda1 | 1 | 1 | primary | 1 |
+--------------+--------------+--------------+--------------+--------------+
| /dev/hda2 | 1 | 1 | primary | 2 |
+--------------+--------------+--------------+--------------+--------------+
| /dev/hda3 | 1 | 1 | primary | 3 |
+--------------+--------------+--------------+--------------+--------------+
| /dev/hda4 | 1 | 1 | swap | NA |
+--------------+--------------+--------------+--------------+--------------+
| /dev/hdb1 | 1 | 2 | primary | 1 |
+--------------+--------------+--------------+--------------+--------------+
| /dev/hdb2 | 1 | 2 | primary | 2 |
+--------------+--------------+--------------+--------------+--------------+
| /dev/hdb3 | 1 | 2 | primary | 3 |
+--------------+--------------+--------------+--------------+--------------+
| /dev/hdb4 | 1 | 2 | primary | 4 |
+--------------+--------------+--------------+--------------+--------------+
Once a drive has been partitioned, the partitions will represented as
numbers on the end of the names. For example, the second partition on the
second drive will be /dev/hdb2. The partition type (primary) is listed in the
table above for clarity, although the concept is not explained until Section
3.3.
Table 4. SCSI Drives
+--------------+--------------+--------------+--------------+--------------+
| drive name | drive | drive number |partition type| partition |
| | controller | | | number |
+--------------+--------------+--------------+--------------+--------------+
| /dev/sda1 | 1 | 6 | primary | 1 |
+--------------+--------------+--------------+--------------+--------------+
| /dev/sda2 | 1 | 6 | primary | 2 |
+--------------+--------------+--------------+--------------+--------------+
| /dev/sda3 | 1 | 6 | primary | 3 |
+--------------+--------------+--------------+--------------+--------------+
SCSI drives follow a similar pattern; They are represented by 'sd' instead
of 'hd'. The first partition of the second SCSI drive would therefore be /dev
/sdb1. In the table above, the drive number is arbitraily chosen to be 6 to
introduce the idea that SCSI ID numbers do not map onto device names under
linux.
-----------------------------------------------------------------------------
2.1.2. Name Assignment
Under (Sun) Solaris and (SGI) IRIX, the device name given to a SCSI drive
has some relationship to where you plug it in. Under linux, there is only
wailing and gnashing of teeth.
Before
SCSI ID #2 SCSI ID #5 SCSI ID #7 SCSI ID #8
/dev/sda /dev/sdb /dev/sdc /dev/sdd
After
SCSI ID #2 SCSI ID #7 SCSI ID #8
/dev/sda /dev/sdb /dev/sdc
SCSI drives have ID numbers which go from 1 through 15. Lower SCSI ID
numbers are assigned lower-order letters. For example, if you have two drives
numbered 2 and 5, then #2 will be /dev/sda and #5 will be /dev/sdb. If you
remove either, all the higher numbered drives will be renamed the next time
you boot up.
If you have two SCSI controllers in your linux box, you will need to
examine the output of /bin/dmesg in order to see what name each drive was
assigned. If you remove one of two controllers, the remaining controller
might have all its drives renamed. Grrr...
There are two work-arounds; both involve using a program to put a label on
each partition (see Section 6). The label is persistent even when the device
is physically moved. You then refer to the partition directly or indirectly
by label.
-----------------------------------------------------------------------------
2.1.3. Logical Partitions
Table 5. Logical Partitions
+--------------+--------------+--------------+--------------+--------------+
| drive name | drive | drive number |partition type| partition |
| | controller | | | number |
+--------------+--------------+--------------+--------------+--------------+
| /dev/hdb1 | 1 | 2 | primary | 1 |
+--------------+--------------+--------------+--------------+--------------+
| /dev/hdb2 | 1 | 2 | extended | NA |
+--------------+--------------+--------------+--------------+--------------+
| /dev/hda5 | 1 | 2 | logical | 2 |
+--------------+--------------+--------------+--------------+--------------+
| /dev/hdb6 | 1 | 2 | logical | 3 |
+--------------+--------------+--------------+--------------+--------------+
The table above illustrates a mysterious jump in the name assignments. This
is due to the use of logical partitions (see Section 3.4, which always start
with 5, for reasons explained later.
This is all you have to know to deal with linux disk devices. For the sake
of completeness, see Kristian's discussion of device numbers below.
-----------------------------------------------------------------------------
2.2. Device numbers
The only important thing with a device file are its major and minor device
numbers, which are shown instead of the file size:
$ ls -l /dev/hda
Table 6. Device file attributes
+-----------+--------+--------+--------+---------+---------+--------+--------+
|brw-rw---- | 1 | root | disk | 3, | 0 | Jul 18 |/dev/hda|
| | | | | | | 1994 | |
+-----------+--------+--------+--------+---------+---------+--------+--------+
|permissions| | owner | group | major | minor | date | device |
| | | | | device | device | | name |
| | | | | number | number | | |
+-----------+--------+--------+--------+---------+---------+--------+--------+
When accessing a device file, the major number selects which device driver is
being called to perform the input/output operation. This call is being done
with the minor number as a parameter and it is entirely up to the driver how
the minor number is being interpreted. The driver documentation usually
describes how the driver uses minor numbers. For IDE disks, this
documentation is in /usr/src/linux/Documentation/ide.txt. For SCSI disks,
one would expect such documentation in /usr/src/linux/Documentation/scsi.txt
, but it isn't there. One has to look at the driver source to be sure ( /usr/
src/linux/driver/scsi/sd.c:184-196). Fortunately, there is Peter Anvin's list
of device numbers and names in /usr/src/linux/Documentation/devices.txt; see
the entries for block devices, major 3, 22, 33, 34 for IDE and major 8 for
SCSI disks. The major and minor numbers are a byte each and that is why the
number of partitions per disk is limited.
-----------------------------------------------------------------------------
3. Partition Types
3.1. Partition Types
A partition is labeled to host a certain kind of file system (not to be
confused with a volume label (see Section 6)). Such a file system could be
the linux standard ext2 file system or linux swap space, or even foreign file
systems like (Microsoft) NTFS or (Sun) UFS. There is a numerical code
associated with each partition type. For example, the code for ext2 is 0x83
and linux swap is 0x82. To see a list of partition types and their codes,
execute /sbin/sfdisk -T
-----------------------------------------------------------------------------
3.2. Foreign Partition Types
The partition type codes have been arbitrarily chosen (you can't figure out
what they should be) and they are particular to a given operating system.
Therefore, it is theoretically possible that if you use two operating systems
with the same hard drive, the same code might be used to designate two
different partition types. OS/2 marks its partitions with a 0x07 type and so
does Windows NT's NTFS. MS-DOS allocates several type codes for its various
flavors of FAT file systems: 0x01, 0x04 and 0x06 are known. DR-DOS used 0x81
to indicate protected FAT partitions, creating a type clash with Linux/Minix
at that time, but neither Linux/Minix nor DR-DOS are widely used any more.
OS/2 marks its partitions with a 0x07 type and so does Windows NT's NTFS.
MS-DOS allocates several type codes for its various flavors of FAT file
systems: 0x01, 0x04 and 0x06 are known. DR-DOS used 0x81 to indicate
protected FAT partitions, creating a type clash with Linux/Minix at that
time, but neither Linux/Minix nor DR-DOS are widely used any more.
-----------------------------------------------------------------------------
3.3. Primary Partitions
The number of partitions on an Intel-based system was limited from the very
beginning: The original partition table was installed as part of the boot
sector and held space for only four partition entries. These partitions are
now called primary partitions.
-----------------------------------------------------------------------------
3.4. Logical Partitions
One primary partition of a hard drive may be subpartitioned. These are
logical partitions. This effectively allows us to skirt the historical four
partition limitation.
The primary partition used to house the logical partitions is called an
extended partition and it has its own file system type (0x05). Unlike primary
partitions, logical partitions must be contiguous. Each logical partition
contains a pointer to the next logical partition, which implies that the
number of logical partitions is unlimited. However, linux imposes limits on
the total number of any type of partition on a drive, so this effectively
limits the number of logical partitions. This is at most 15 partitions total
on an SCSI disk and 63 total on an IDE disk.
-----------------------------------------------------------------------------
3.5. Swap Partitions
Every process running on your computer is allocated a number of blocks of
RAM. These blocks are called pages. The set of in-memory pages which will be
referenced by the processor in the very near future is called a "working
set." Linux tries to predict these memory accesses (assuming that recently
used pages will be used again in the near future) and keeps these pages in
RAM if possible.
If you have too many processes running on a machine, the kernel will try to
free up RAM by writing pages to disk. This is what swap space is for. It
effectively increases the amount of memory you have available. However, disk
I/O is about a hundred times slower than reading from and writing to RAM.
Consider this emergency memory and not extra memory.
If memory becomes so scarce that the kernel pages out from the working set
of one process in order to page in for another, the machine is said to be
thrashing. Some readers might have inadvertenly experienced this: the hard
drive is grinding away like crazy, but the computer is slow to the point of
being unusable. Swap space is something you need to have, but it is no
substitute for sufficient RAM. See the discussion in Section 4.4 for tips on
determining the size of swap space you need.
-----------------------------------------------------------------------------
4. Partitioning requirements
4.1. What Partitions do I need?
For the Boot Drive: If you want to boot your operating system from the drive
you are about to partition, you will need:
* A primary partition
* One or more swap partitions
* Zero or more primary/logical partitions
For any other drive:
* One or more primary/logical partitions
* Zero or more swap partitions
-----------------------------------------------------------------------------
4.2. Discussion:
Boot Partition:
Your boot partition ought to be a primary partition, not a logical
partition. This will ease recovery in case of disaster, but it is not
technically necessary. It must be of type 0x83 "Linux native". If you are
using a version of lilo before 21-3 (ie, from the 1990s), your boot
partition must be contained within the first 1024 cylinders of the drive.
(Typically, the boot partition need only contain the kernel image.)
If you have more than one boot partition (from other OSs, for example,)
keep them all in the first 1024 cylinders (All DOS partitions must be
within the first 1024). If you are using a modern version of lilo, or a
means other than lilo to load your kernel (for example, a boot disk or
the LOADLIN.EXE MS-DOS based Linux loader), the partition can be
anywhere. See the [http://www.tldp.org/HOWTO/Large-Disk-HOWTO.html]
Large-disk HOWTO for details.
Swap Partition:
Unless you swap to files (see Section 9.2) you will need a dedicated
swap partition. It must be of type 0x82 "Linux swap". It may be
positioned anywhere on the disk (but see Section 4.4.3). Either a primary
or logical partition can be used for swap. More than one swap partition
can exist on a drive. 8 total (across drives) are permitted. See notes on
swap size below (Section 4.4).
Logical Partition:
A single primary partition must be used as a container (extended
partition) for the logical partitions. The extended partition can go
anywhere on the disk. The logical partitions must be contiguous, but
needn't fill the extended partition.
-----------------------------------------------------------------------------
4.3. File Systems
4.3.1. Which file systems need their own partitions?
Everything in your linux file system can go in the same (single) partition.
However, there are circumstances when you may want to restrict the growth of
certain file systems. For example, if your mail spool was in the same
partition as your root fs and it filled the remaining space in the partition,
your computer would basically hang.
/var
This fs contains spool directories such as those for mail and printing.
In addition, it contains the error log directory. If your machine is a
server and develops a chronic error, those msgs can fill the partition.
Server computers ought to have /var in a different partition than /.
/usr
This is where most executable binaries go. In addition, the kernel source
tree goes here, and much documentation.
/tmp
Some programs write temporary data files here. Usually, they are quite
small. However, if you run computationally intensive jobs, like science
or engineering applications, hundreds of megabytes could be required for
brief periods of time. In this case, keep /tmp in a different partition
than /.
/home
This is where users home directories go. If you do not impose quotas on
your users, this ought to be in its own partition.
/boot
This is where your kernel images go. See discussion above for placement
on old systems.
-----------------------------------------------------------------------------
4.3.2. File lifetimes and backup cycles as partitioning criteria
With ext2, partitioning decisions should be governed by backup considerations
and to avoid external fragmentation Section 10.4 from different file
lifetimes.
Files have lifetimes. After a file has been created, it will remain some time
on the system and then be removed. File lifetime varies greatly throughout
the system and is partly dependent on the pathname of the file. For example,
files in /bin, /sbin, /usr/sbin, /usr/bin and similar directories are likely
to have a very long lifetime: many months and above. Files in /home are
likely to have a medium lifetime: several weeks or so. File in /var are
usually short lived: Almost no file in /var/spool/news will remain longer
than a few days, files in /var/spool/lpd measure their lifetime in minutes or
less.
For backup it is useful if the amount of daily backup is smaller than the
capacity of a single backup medium. A daily backup can be a complete backup
or an incremental backup.
You can decide to keep your partition sizes small enough that they fit
completely onto one backup medium (choose daily full backups). In any case a
partition should be small enough that its daily delta (all modified files)
fits onto one backup medium (choose incremental backup and expect to change
backup media for the weekly/monthly full dump - no unattended operation
possible).
Your backup strategy depends on that decision.
When planning and buying disk space, remember to set aside a sufficient
amount of money for backup! Unbackuped data is worthless! Data reproduction
costs are much higher than backup costs for virtually everyone!
For performance it is useful to keep files of different lifetimes on
different partitions. This way the short lived files on the news partition
may be fragmented very heavily. This has no impact on the performance of the
/ or /home partition.
-----------------------------------------------------------------------------
4.4. Swap Partitions
4.4.1. How large should my swap space be?
Conventional wisdom creates swap space equal to the amount of RAM.
But keep in mind that this is just a rule of thumb. It is easily possible
to create scenarios where programs have extremely large or extremely small
working sets (see Section 3.5). For example, a simulation program with a
large data set that is accessed in a very random fashion would have almost no
noticeable locality of reference in its data segment, so its working set
would be quite large.
On the other hand, a graphics program with many simultaneously opened
JPEGs, all but one iconified, would have a very large data segment. But image
transformations are all done on one single image, most of the memory occupied
by the program is not accessed. The same is true for an editor with many
editor windows where only one window is being modified at a time. These
programs have - if they are designed properly - a very high locality of
reference and large parts of them can be kept swapped out without too severe
performance impact. A user who never never quits programs once launched would
want a lot of swap space for the same reason.
Servers typically are configured with more swap space than their desktop
counterparts. Even though a given amount of swap is sufficient for its
operations, the server might come under transient heavy loads which cause it
to page out at a high rate. Some administrators prefer this to the server
crashing altogether. In these cases, swap might be several times the size of
ram.
-----------------------------------------------------------------------------
4.4.2. How large can my swap space be?
Currently, the maximum size of a swap partition is architecture-dependent.
For i386, m68k, ARM and PowerPC, it is "officially" 2Gb. It is 128Gb on
alpha, 1Gb on sparc, and 3Tb on sparc64. An opteron on the 2.6 kernel can
write to a 16 Tb swap partition. For linux kernels 2.1 and earlier, the limit
is 128Mb. The partition may be larger than 128 MB, but excess space is never
used. If you want more than 128 MB of swap for a 2.1 and earlier kernel, you
have to create multiple swap partitions (8 max). After 2.4, 32 swap areas are
"officially" possible. See setting up swap for details.
footnote: "official" max swap size: With kernel 2.4, the limit is 64 swap
spaces at a maximum of 64Gb each, although this is not reflected in the man
page for mkswap. With the 64 bit opteron on the 2.6 kernel, 128 swap areas
are permitted, each a whopping 16 Tb! (thanks to Peter Chubb for the
calculation)
-----------------------------------------------------------------------------
4.4.3. Where should I put my swap space?
The short answer is anywhere is fine. However, if you are interested in
extracting as much speed as possible, there are two basic strategies (other
than buying more RAM).
* Split the swap space across multiple drives, or at least on the drive you
write to least.
* Put each swap partition on the outer tracks.
Here are the considerations:
* If you have a disk with many heads and one with less heads and both are
identical in other parameters, the disk with many heads will be faster.
Reading data from different heads is fast, since it is purely electronic.
Reading data from different tracks is slow, since it involves physically
moving the head.
It follows then that writing swap on a separate drive will be faster
than moving the head back and forth on a single drive.
* Placement: Older disks have the same number of sectors on all tracks.
With these disks it will be fastest to put your swap in the middle of the
disks, assuming that your disk head will move from a random track towards
the swap area.
* Newer disks use ZBR (zone bit recording). They have more sectors on the
outer tracks. With a constant number of rpms, this yields a far greater
performance on the outer tracks than on the inner ones. Put your swap on
the fast tracks. (In general, low-numbered cylinders are associated low
partition numbers. However, see Kristian's more recent [http://lissot.net
/partition/mapping.html] comments on this issue. -Tony)
* Usage: Of course your disk head will not move randomly. If you have swap
space in the middle of a disk between a constantly busy home partition
and an almost unused archive partition, you would be better of if your
swap were near the home partition for even shorter head movements. You
would be even better off, if you had your swap on another otherwise
unused disk, though.
* Striping: Speed can be increased by writing to multiple swap areas
simultaneously. Swap spaces with the same priority will be written to
like a RAID. See Section 9.3.
Summary: Put your swap on a fast disk with many heads that is not busy doing
other things. If you have multiple disks: Split swap and scatter it over all
your disks or even different controllers.
-----------------------------------------------------------------------------
5. Partitioning with fdisk
This section shows you how to actually partition your hard drive with the
fdisk utility. Linux allows only 4 primary partitions. You can have a much
larger number of logical partitions by sub-dividing one of the primary
partitions. Only one of the primary partitions can be sub-divided.
Examples:
1. Four primary partitions (see Section 5.2)
2. Mixed primary and logical partitions (see Section 5.3)
-----------------------------------------------------------------------------
5.1. fdisk usage
fdisk is started by typing (as root) fdisk device at the command prompt.
device might be something like /dev/hda or /dev/sda (see Section 2.1.1). The
basic fdisk commands you need are:
p print the partition table
n create a new partition
d delete a partition
q quit without saving changes
w write the new partition table and exit
Changes you make to the partition table do not take effect until you issue
the write (w) command. Here is a sample partition table:
Disk /dev/hdb: 64 heads, 63 sectors, 621 cylinders
Units = cylinders of 4032 * 512 bytes
Device Boot Start End Blocks Id System
/dev/hdb1 * 1 184 370912+ 83 Linux
/dev/hdb2 185 368 370944 83 Linux
/dev/hdb3 369 552 370944 83 Linux
/dev/hdb4 553 621 139104 82 Linux swap
The first line shows the geometry of your hard drive. It may not be
physically accurate, but you can accept it as though it were. The hard drive
in this example is made of 32 double-sided platters with one head on each
side (probably not true). Each platter has 621 concentric tracks. A
3-dimensional track (the same track on all disks) is called a cylinder. Each
track is divided into 63 sectors. Each sector contains 512 bytes of data.
Therefore the block size in the partition table is 64 heads * 63 sectors *
512 bytes er...divided by 1024. (See 4 for discussion on problems with this
calculation.) The start and end values are cylinders.
-----------------------------------------------------------------------------
5.2. Four primary partitions
The overview:
Decide on the size of your swap space (see Section 4.4) and where it ought
to go (see Section 4.4.3). Divide up the remaining space for the three other
partitions.
Example:
I start fdisk from the shell prompt:
# fdisk /dev/hdb
which indicates that I am using the second drive on my IDE controller. (See
Section 2.1.) When I print the (empty) partition table, I just get
configuration information.
Command (m for help): p
Disk /dev/hdb: 64 heads, 63 sectors, 621 cylinders
Units = cylinders of 4032 * 512 bytes
I knew that I had a 1.2Gb drive, but now I really know: 64 * 63 * 512 * 621 =
1281982464 bytes. I decide to reserve 128Mb of that space for swap, leaving
1153982464. If I use one of my primary partitions for swap, that means I have
three left for ext2 partitions. Divided equally, that makes for 384Mb per
partition. Now I get to work.
Command (m for help): n
Command action
e extended
p primary partition (1-4)
p
Partition number (1-4): 1
First cylinder (1-621, default 1):<RETURN>
Using default value 1
Last cylinder or +size or +sizeM or +sizeK (1-621, default 621): +384M
Next, I set up the partition I want to use for swap:
Command (m for help): n
Command action
e extended
p primary partition (1-4)
p
Partition number (1-4): 2
First cylinder (197-621, default 197):<RETURN>
Using default value 197
Last cylinder or +size or +sizeM or +sizeK (197-621, default 621): +128M
Now the partition table looks like this:
Device Boot Start End Blocks Id System
/dev/hdb1 1 196 395104 83 Linux
/dev/hdb2 197 262 133056 83 Linux
I set up the remaining two partitions the same way I did the first. Finally,
I make the first partition bootable:
Command (m for help): a
Partition number (1-4): 1
And I make the second partition of type swap:
Command (m for help): t
Partition number (1-4): 2
Hex code (type L to list codes): 82
Changed system type of partition 2 to 82 (Linux swap)
Command (m for help): p
The end result:
Disk /dev/hdb: 64 heads, 63 sectors, 621 cylinders
Units = cylinders of 4032 * 512 bytes
Device Boot Start End Blocks Id System
/dev/hdb1 * 1 196 395104+ 83 Linux
/dev/hdb2 197 262 133056 82 Linux swap
/dev/hdb3 263 458 395136 83 Linux
/dev/hdb4 459 621 328608 83 Linux
Finally, I issue the write command (w) to write the table on the disk.
Side topics:
* Section 10.2
* Section 10.1
* Section 10.3
-----------------------------------------------------------------------------
5.3. Mixed primary and logical partitions
The overview: create one use one of the primary partitions to house all the
extra partitions. Then create logical partitions within it. Create the other
primary partitions before or after creating the logical partitions.
Example:
I start fdisk from the shell prompt:
# fdisk /dev/sda
which indicates that I am using the first drive on my SCSI chain. (See
Section 2.1.)
First I figure out how many partitions I want. I know my drive has a 183Gb
capacity and I want 26Gb partitions (because I happen to have back-up tapes
that are about that size).
183Gb / 26Gb = ~7
so I will need 7 partitions. Even though fdisk accepts partition sizes
expressed in Mb and Kb, I decide to calculate the number of cylinders that
will end up in each partition because fdisk reports start and stop points in
cylinders. I see when I enter fdisk that I have 22800 cylinders.
> The number of cylinders for this disk is set to 22800. There is
> nothing wrong with that, but this is larger than 1024, and could in
> certain setups cause problems with: 1) software that runs at boot
> time (e.g., LILO) 2) booting and partitioning software from other
> OSs (e.g., DOS FDISK, OS/2 FDISK)
So, 22800 total cylinders divided by seven partitions is 3258 cylinders. Each
partition will be about 3258 cylinders long. I ignore the warning msg because
this is not my boot drive (Section 4).
Since I have 4 primary partitions, 3 of them can be 3258 long. The extended
partition will have to be (4 * 3258), or 13032, cylinders long in order to
contain the 4 logical partitions.
I enter the following commands to set up the first of the 3 primary
partitions (stuff I type is bold ):
Command (m for help): n
Command action
e extended
p primary partition (1-4)
p
Partition number (1-4): 1
First cylinder (1-22800, default 1): <RETURN>
Using default value 1
Last cylinder or +size or +sizeM or +sizeK (1-22800, default 22800): 3258
The last partition is the extended partition:
Partition number (1-4): 4
First cylinder (9775-22800, default 9775): <RETURN>
Using default value 9775
Last cylinder or +size or +sizeM or +sizeK (9775-22800, default 22800): <RETURN>
Using default value 22800
The result, when I issue the print table command is:
/dev/sda1 1 3258 26169853+ 83 Linux
/dev/sda2 3259 6516 26169885 83 Linux
/dev/sda3 6517 9774 26169885 83 Linux
/dev/sda4 9775 22800 104631345 5 Extended
Next I segment the extended partition into 4 logical partitions, starting
with the first logical partition, into 3258-cylinder segments. The logical
partitions automatically start from /dev/sda5.
Command (m for help): n
First cylinder (9775-22800, default 9775): <RETURN>
Using default value 9775
Last cylinder or +size or +sizeM or +sizeK (9775-22800, default 22800): 13032
The end result is:
Device Boot Start End Blocks Id System
/dev/sda1 1 3258 26169853+ 83 Linux
/dev/sda2 3259 6516 26169885 83 Linux
/dev/sda3 6517 9774 26169885 83 Linux
/dev/sda4 9775 22800 104631345 5 Extended
/dev/sda5 9775 13032 26169853+ 83 Linux
/dev/sda6 13033 16290 26169853+ 83 Linux
/dev/sda7 16291 19584 26459023+ 83 Linux
/dev/sda8 19585 22800 25832488+ 83 Linux
Finally, I issue the write command (w) to write the table on the disk. To
make the partitions usable, I will have to format (Section 10.1) each
partition and then mount (Section 10.3) it.
-----------------------------------------------------------------------------
5.4. Submitted Examples
I'd like to submit my partition layout, because it works well with any
distribution of Linux (even big RPM based ones). I have one hard drive that
... is 10 gigs, exactly. Windows can't see above 9.3 gigs of it, but Linux
can see it all, and use it all. It also has much more than 1024 cylenders.
Table 7. Partition layout example
+-------------------+--------------------+-------------------+
|Partition |Mount point |Size |
+-------------------+--------------------+-------------------+
|/dev/hda1 |/boot |(15 megs) |
+-------------------+--------------------+-------------------+
|/dev/hda2 |windows 98 partition|(2 gigs) |
+-------------------+--------------------+-------------------+
|/dev/hda3 |extended |(N/A) |
+-------------------+--------------------+-------------------+
|/dev/hda5 |swap space |(64 megs) |
+-------------------+--------------------+-------------------+
|/dev/hda6 |/tmp |(50 megs) |
+-------------------+--------------------+-------------------+
|/dev/hda7 |/ |(150 megs) |
+-------------------+--------------------+-------------------+
|/dev/hda8 |/usr |(1.5 gigs) |
+-------------------+--------------------+-------------------+
|/dev/hda9 |/home |(rest of drive) |
+-------------------+--------------------+-------------------+
I test new kernels for the USB mass storage, so that explains the large /boot
partition. I install LILO into the MBR, and by default I boot windows (I'm
not the only one to use this computer).
I also noticed that you don't have any REAL examples of partition tables,
and for newbies I HIGHLY suggest putting quite a few up. I'm freshly out of
the newbie stage, and partitioning was what messed me up the most.
[mailto:valkor@qx.net] Valkor
-----------------------------------------------------------------------------
6. Labels
In linux, hard drives are referred to as devices, and devices are pseudo
files in /dev. For example, the first partition of the second lowest numbered
SCSI drive is /dev/sdb1. If the drive referred to as /dev/sda is removed from
the chain, then the latter partition is automatically renamed /dev/sda1 at
reboot.
-----------------------------------------------------------------------------
6.1. Volume Labels
Volume labels make it possible for partitions to retain a consistent name
regardless of where they are connected, and regardless of whatever else is
connected. Labels are not mandatory for a linux volume. Each can be a maximum
of 16 characters long.
There are three tools to make volume labels: mke2fs, tune2fs and e2label.
-----------------------------------------------------------------------------
6.1.1. Simple Invocation
e2label /dev/hdb1 pubsw
tune2fs -L pubsw /dev/hdb1
Either of thse two commands will label the first partition of the second
drive "pubsw". That label stays with that particular partition, even if the
drive is moved to another controller or even another computer.
mke2fs pubsw /dev/hdb1
mke2fs -L pubsw /dev/hdb1
will do the same thing as the first two commands - after they make the file
system. This means that either of these last two commands will delete any
existing data in the partition.
-----------------------------------------------------------------------------
6.1.2. How to Use
Here is a sample fstab. This is a text file located in /etc, which is
usually set up during the installation of the operating system. it describes
where each partition wil be mounted, and how it will be mounted. It can be
modified by you, either through a utility or manually, when you add/remove
devices.
LABEL=/ / ext3 defaults 1 1
LABEL=/boot /boot ext2 defaults 1 2
none /dev/pts devpts gid=5,mode=620 0 0
none /dev/shm tmpfs defaults 0 0
LABEL=HOME /home ext3 defaults 1 2
none /proc proc defaults 0 0
none /sys sysfs defaults 0 0
LABEL=/usr /usr ext3 defaults 1 2
/dev/hdc1 /k-space ext3 defaults 1 2
/dev/hda6 swap swap defaults 0 0
/dev/hdd /media/cdrecorder auto pamconsole,ro,exec,noauto,managed 0 0
/dev/fd0 /media/floppy auto pamconsole,exec,noauto,managed 0 0
The leftmost column lists devices and the second column lists mount points.
This example contains a mixture of devices and labels. The master drive of
the second controller is always mounted on /k-space. The partition labeled
"HOME" is always mounted on /home, regardless of which drive it is on or
which partition number it has. Notice that it is permissible to use mount
points as labels, such as "/usr"
-----------------------------------------------------------------------------
6.2. Device Labels
devlabel is a script which creates symbolic links to devices. For example,
devlabel -d /dev/hdb1 -s /dev/home
will create a link from /dev/hdb1 to /dev/home. Crucially, it stores a
unique identifier for the hardware that was on /dev/hdb1 and stores that
identifier along with the link name that you specified in /etc/sysconfig/
devlabel. If the hardware is later moved to /dev/hdc1, its unique identifier
will be queried (using /usr/bin/partition_uuid), matched to its entry in /etc
/sysconfig/devlabel, and again linked to /dev/home.
-----------------------------------------------------------------------------
7. Formatting an ext2/3 partition
When a hard drive is partitioned, it is mapped into sections, but the
sections are empty. It is like a newly constructed library; shelves, signs,
and a card catalogue system must be put in place before the books are put
away.
The organizational structure inside a partition is called a file system.
With Linux, the standard file system is ext2 and ext3. The ext3 file system
is ext2, plus a log of disk writes called a journal. The journal allows the
system to recover quickly from accidental power outages, among other things.
The principal tool for making an ext2/3 file system in a partition is
mke2fs. It is usually found in /sbin. mkfs.ext2 and mkfs.ext3 are frontends
which pass specific options to mke2fs.
-----------------------------------------------------------------------------
.1. Simple Invocation
mke2fs /dev/hdb1
mkfs.ext2 /dev/hdb1
both of which make an ext2 file system on the first partition of the second
drive, and
mke2fs -j /dev/hdb1
mkfs.ext3 /dev/hdb1
make an ext3 file system.
-----------------------------------------------------------------------------
.2. Reserved blocks
The -m option is probably the one of most use to non-experts. If the file
system becomes filled and there is no more space to write, it is basically
unusable because the operating system is constantly writing to disk. By
default, five percent of the partition is reserved for use by the root user.
This allows root to conduct administrative activities on the partition and
perhaps move some data off. However, this is most critical when the partition
contains / or home directories. For pure data partitions, this is just lost
space. Five percent of a 250Gb partition is 12.5 Gb. Especially in the case
of large partitions, it is safe to set the reserved space to the minimum,
which is one percent.
mkfs.ext3 -m 1/dev/hdb1
creates a file system with only 1% of its space reserved for the root user.
tune2fs -m can be used to adjust the reserved blocks after data is loaded on
the partition.
-----------------------------------------------------------------------------
8. Recovering a Deleted Partition Table
Below are instructions for manually recovering a deleted partition table.
There are utilities such as [http://www.stud.uni-hannover.de/user/76201/
gpart] gpart or TestDisk which can make this task considerably easier. If you
are reading this, however, because you have run out of luck, this is what you
will have to do:
1. Make a partition that is at least as big as your first partition was. You
can make it larger than the original partition by any amount. If you
underestimate, there will be much wailing and gnashing of teeth.
Command (m for help): n
Command action
e extended
p primary partition (1-4)
p
Partition number (1-4): 1
First cylinder (1-23361, default 1): <RETURN>
Using default value 1
Last cylinder or +size or +sizeM or +sizeK (1-22800, default 22800): 13032
Command (m for help): w
2. Run dumpe2fs on the first partition and grep out the block count.
Example:
% dumpe2fs /dev/sda1 | grep "Block count:"
Block count: 41270953
If you are uncertain about this value, repeat Step 1 with a bigger
partition size. If the block count changes, then you underestimated the
size of the original partition. Repeat Step 1 until you get a stable
block count.
3. Remove the partition you just created
Command (m for help): d
Partition number (1-4): 1
4. Make a new partition with the exact size you got from the block count.
Since you cannot enter block size in fdisk, you need to figure out how
many cylinders to request. Here is the formula:
(number of needed cylinders) = (number of blocks) / (block size)
(block size) = (unit size) / 1024
(unit size) = (number of heads) * (number of sectors/cylinder) * (number of bytes/sector)
Consider the following example, where a hard drive has been partitioned
into four primary partitions of 1, 2, 4, and 8 cylinders.
disk /dev/sda: 16 heads, 63 sectors, 23361 cylinders
Units = cylinders of 1008 * 512 bytes
Device Boot Start End Blocks Id System
/dev/sda1 1 2 976+ 83 Linux
/dev/sda2 3 5 1512 83 Linux
/dev/sda3 6 10 2520 83 Linux
/dev/sda4 11 19 4536 83 Linux
fdisk provides the configuration information I need in the head of the
output. The unit size is 516096 ( 16 heads * 63 sectors/cyl * 512 bytes/
sector ). The block size is 504 ( 516096 / 1024 ). The number of needed
cylinders for the second partition is therefore 3 ( 1512 blocks / 504 ).
The partition table shows that this is indeed the case: the first
cylinder is 3, the second 4, and the last is 5, for a total of three
cylinders. The number of needed cylinders for the third partition is
calculated similarly: 2520 blocks / 504 = 5, which corresponds to blocks
6,7,8,9,10 . Notice that this calculation does not work for the first
partition because the block count is wrong ( 976 instead of 1008 ). The
plus sign indicates that not all the blocks are included in the fdisk
value. When you try the calculation ( 976 / 504 ) you get 1.937. Knowing
that the number of cylinders must be an integer, you can simply round up.
5. Run e2fsck on it to verify that you can read the new partition.
6. Repeat Steps 1-5 on remaining partitions.
Remount your partitions. Amazingly, all of your data will be there.
Credit goes to: Mike Vevea, jedi sys admin, for providing the basic
strategy.
-----------------------------------------------------------------------------
9. Setting Up Swap Space
9.1. Swap Files
Normally, there are only two steps to setting up swap space, creating the
partition and adding it to /etc/fstab. A typical fstab entry for a swap
partition at /dev/hda6 would look like this:
/dev/hda6 swap swap defaults 0 0
The next time you reboot, the initialization scripts will activate it
automatically and there's nothing more to be done.
However, if you want to make use of it right away, you'll need to activate
it maually. As root, type:
mkswap -f /dev/hda6
swapon /dev/hda6
-----------------------------------------------------------------------------
9.2. Swap Files
There might be times when you've run out of swap space and it is not
practical to repartition a drive or add a new one. In this case, you can use
a regular file in an ordinary partition. All you have to do is create a file
of the size you want
dd if=/dev/zero of=/var/my_swap bs=1024 count=131072
and activate it
mkswap -f /var/my_swap
swapon /var/my_swap
This invocation creates a file called my_swap in /var. It is 128 Mb long
(128 x 1024 = 131072). Initially, it is filled with zeros. However, mkswap
marks it as swap space and swapon tells the kernel to start using it as swap
space. When you are done with it,
swapoff /var/my_swap
rm /var/my_swap
-----------------------------------------------------------------------------
9.3. Multiple Swap Areas
More than one swap partition can be used on the same system. Consider an
example fstab where there is a single swap partition:
/dev/hda5 / ext3 defaults 1 1
/dev/hda1 /boot ext2 defaults 1 2
none /dev/pts devpts gid=5,mode=620 0 0
none /proc proc defaults 0 0
/dev/hda7 /usr ext3 defaults 1 2
/dev/hda6 swap swap defaults 0 0
Imagine replacing the entry for the swap partition with these three lines:
/dev/hda6 none swap sw,pri=3 0 0
/dev/hdb2 none swap sw,pri=2 0 0
/dev/hdc2 none swap sw,pri=1 0 0
This configuration would cause the kernel to use /dev/hda6 first. it has
the highest priority assigned to it (pri=3). The maximum priority can be
32767 and the lowest 0. If that space were to max out, the kernel would start
using /dev/hdb2, and on to /dev/hdc2 after that. Why such a configuration?
Imagine that the newest (fastest) drives are given the highest priority. This
will minimize speed loss as swap space usage grows.
It is possible to write to all three simulataneously. If each has the same
priority, the kernel will write to them much like a RAID, with commensurate
speed increases.
/dev/hda6 none swap sw,pri=3 0 0
/dev/hdb2 none swap sw,pri=3 0 0
/dev/hdc2 none swap sw,pri=3 0 0
Notice that these three partitions are on separate drives, which is ideal
in terms of speed enhancement.
-----------------------------------------------------------------------------
10. Appendix
10.1. Formating Partitions
At the shell prompt, I begin making the file systems on my partitions.
Continuing with the example in (see Section 5.3), this is:
# mke2fs /dev/sda1
I need to do this for each of my partitions, but not for /dev/sda4 (my
extended partition). Linux supports types of file systems other than ext2.
You can find out what kinds your kernel supports by looking in: /usr/src/
linux/include/linux/fs.h
The most common file systems can be made with programs in /sbin that start
with "mk" like mkfs.msdos and mke2fs.
-----------------------------------------------------------------------------
10.2. Activating Swap Space
To set up a swap partition:
# mkswap -f /dev/hda5
To activate the swap area:
# swapon /dev/hda5
Normally, the swap area is activated by the initialization scripts at boot
time.
-----------------------------------------------------------------------------
10.3. Mounting Partitions
Mounting a partition means attaching it to the linux file system. To mount
a linux partition:
# mount -t ext2 /dev/sda1 /opt
-t ext2
File system type. Other types you are likely to use are:
+ ext3 (journaling sile system based on ext2)
+ msdos (DOS)
+ hfs (mac)
+ iso9660 (CDROM)
+ nfs (network file system)
/dev/sda1
Device name. Other device names you are likely to use:
+ /dev/hdb2 (second partition in second IDE drive)
+ /dev/fd0 (floppy drive A)
+ /dev/cdrom (CDROM)
/opt
mount point. This is where you want to "see" your partition. When you
type ls /opt, you can see what is in /dev/sda1. If there are already some
directories and/or files under /opt, they will be invisible after this
mount command.
-----------------------------------------------------------------------------
10.4. Some facts about file systems and fragmentation
Disk space is administered by the operating system in units of blocks and
fragments of blocks. In ext2, fragments and blocks have to be of the same
size, so we can limit our discussion to blocks.
Files come in any size. They don't end on block boundaries. So with every
file a part of the last block of every file is wasted. Assuming that file
sizes are random, there is approximately a half block of waste for each file
on your disk. Tanenbaum calls this "internal fragmentation" in his book
"Operating Systems".
You can guess the number of files on your disk by the number of allocated
inodes on a disk. On my disk
# df -i
Filesystem Inodes IUsed IFree %IUsed Mounted on
/dev/hda3 64256 12234 52022 19% /
/dev/hda5 96000 43058 52942 45% /var
there are about 12000 files on / and about 44000 files on /var. At a block
size of 1 KB, about 6+22 = 28 MB of disk space are lost in the tail blocks of
files. Had I chosen a block size of 4 KB, I had lost 4 times this space.
Data transfer is faster for large contiguous chunks of data, though. That's
why ext2 tries to preallocate space in units of 8 contigous blocks for
growing files. Unused preallocation is released when the file is closed, so
no space is wasted.
Noncontiguous placement of blocks in a file is bad for performance, since
files are often accessed in a sequential manner. It forces the operating
system to split a disk access and the disk to move the head. This is called
"external fragmentation" or simply "fragmentation" and is a common problem
with MS-DOS file systems. In conjunction with the abysmal buffer cache used
by MS-DOS, the effects of file fragmentation on performance are very
noticeable. DOS users are accustomed to defragging their disks every few
weeks and some have even developed some ritualistic beliefs regarding
defragmentation.
None of these habits should be carried over to Linux and ext2. Linux native
file systems do not need defragmentation under normal use and this includes
any condition with at least 5% of free space on a disk. There is a
defragmentation tool for ext2 called defrag, but users are cautioned against
casual use. A power outage during such an operation can trash your file
system. Since you need to back up your data anyway, simply writing back from
your copy will do the job.
The MS-DOS file system is also known to lose large amounts of disk space
due to internal fragmentation. For partitions larger than 256 MB, DOS block
sizes grow so large that they are no longer useful (This has been corrected
to some extent with FAT32). Ext2 does not force you to choose large blocks
for large file systems, except for very large file systems in the 0.5 TB
range (that's terabytes with 1 TB equaling 1024 GB) and above, where small
block sizes become inefficient. So unlike DOS there is no need to split up
large disks into multiple partitions to keep block size down.
Use a 1Kb block size if you have many small files. For large partitions,
4Kb blocks are fine.