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

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

2.1. Device names

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

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.

3.1. 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.

4.1. 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

• One or more primary/logical partitions

• Zero or more swap partitions

4.2. Discussion:

4.3. File Systems

4.4. Swap Partitions

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.

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.

Valkor

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.

6.2. 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.

.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.

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.

10.1. 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

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.