Linux Partition HOWTO


3. Partition Types

3.1. Partition Types

A partition is labeled to host a certain kind of file system. 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.

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.

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 prtition 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 very slow compared to reading from and writing to RAM.

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. Expect performance to drop by approximately the ratio between
memory access speed and disk access speed. Swap space is something you need
to have, but it is no substitute for sufficient RAM. See Section 4.4.1 for
tips on determining the size of swap space you need.

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