| Rev | Author | # | Line |
|---|---|---|---|
| 1 | perry | 1 | !!NAME |
| 3 | CraigBox | 2 | mkswap - set up a Linux swap area |
| 1 | perry | 3 | |
| 4 | !!SYNOPSIS | ||
| 3 | CraigBox | 5 | __mkswap [[-c] [[-v__ ''N'' __] [[-f] [[-p __ ''PSZ'' __] __ ''device'' __ [[__ ''size'' __]__ |
| 1 | perry | 6 | |
| 7 | !!DESCRIPTION | ||
| 3 | CraigBox | 8 | __mkswap__ sets up a Linux swap area on a device or in a file. |
| 1 | perry | 9 | |
| 3 | CraigBox | 10 | (After creating the swap area, you need the __swapon__ command to start using it. Usually swap areas are listed in ''/etc/fstab'' so that they can be taken into use at boot time by a __swapon -a__ command in some boot script.) |
| 1 | perry | 11 | |
| 3 | CraigBox | 12 | The ''device'' argument will usually be a disk partition (something like ''/dev/hda4'' or ''/dev/sdb7'' ) but can also be a file. The Linux kernel does not look at partition Id's, but many installation scripts will assume that partitions of hex type 82 (LINUX_SWAP) are meant to be swap partitions. (Warning: Solaris also uses this type. Be careful not to kill your Solaris partitions.) |
| 1 | perry | 13 | |
| 3 | CraigBox | 14 | The ''size'' parameter is superfluous but retained for backwards compatibility. (It specifies the desired size of the swap area in 1024-byte blocks. __mkswap__ will use the entire partition or file if it is omitted. Specifying it is unwise - a typo may destroy your disk.) |
| 1 | perry | 15 | |
| 3 | CraigBox | 16 | The ''PSZ'' parameter specifies the page size to use. It is almost always unnecessary (even unwise) to specify it, but certain old libc versions lie about the page size, so it is possible that __mkswap__ gets it wrong. The symptom is that a subsequent __swapon__ fails because no swap signature is found. Typical values for ''PSZ'' are 4096 or 8192. |
| 1 | perry | 17 | |
| 3 | CraigBox | 18 | Linux knows about two styles of swap areas, old style and new style. The last 10 bytes of the first page of the swap area distinguishes them: old style has `SWAP_SPACE', new style has `SWAPSPACE2' as signature. |
| 1 | perry | 19 | |
| 3 | CraigBox | 20 | In the old style, the rest of this first page was a bit map, with a 1 bit for each usable page of the swap area. Since the first page holds this bit map, the first bit is 0. Also, the last 10 bytes hold the signature. So, if the page size is S, an old style swap area can describe at most 8*(S-10)-1 pages used for swapping. With S=4096 (as on i386), the useful area is at most 133890048 bytes (almost 128 MiB), and the rest is wasted. On an alpha and sparc64, with S=8192, the useful area is at most 535560992 bytes (almost 512 MiB). |
| 1 | perry | 21 | |
| 3 | CraigBox | 22 | The old setup wastes most of this bitmap page, because zero bits denote bad blocks or blocks past the end of the swap space, and a simple integer suffices to indicate the size of the swap space, while the bad blocks, if any, can simply be listed. Nobody wants to use a swap space with hundreds of bad blocks. (I would not even use a swap space with 1 bad block.) In the new style swap area this is precisely what is done. The maximum useful size of a swap area now depends on the architecture. It is roughly 2GiB on i386, PPC, m68k, ARM, 1GiB on sparc, 512MiB on mips, 128GiB on alpha and 3TiB on sparc64. |
| 1 | perry | 23 | |
| 3 | CraigBox | 24 | Note that before 2.1.117 the kernel allocated one byte for each page, while it now allocates two bytes, so that taking a swap area of 2 GiB in use might require 2 MiB of kernel memory. |
| 1 | perry | 25 | |
| 3 | CraigBox | 26 | Presently, Linux allows 8 swap areas. The areas in use can be seen in the file ''/proc/swaps'' (since 2.1.25). |
| 1 | perry | 27 | |
| 3 | CraigBox | 28 | __mkswap__ refuses areas smaller than 10 pages. |
| 1 | perry | 29 | |
| 3 | CraigBox | 30 | If you don't know the page size that your machine uses, you may be able to look it up with "cat /proc/cpuinfo" (or you may not - the contents of this file depend on architecture and kernel version). |
| 1 | perry | 31 | |
| 3 | CraigBox | 32 | To setup a swap file, it is necessary to create that file before initializing it with __mkswap ,__ e.g. using a command like |
| 1 | perry | 33 | |
| 3 | CraigBox | 34 | # dd if=/dev/zero of=swapfile bs=1024 count=65536 %%% |
| 1 | perry | 35 | |
| 3 | CraigBox | 36 | Note that a swap file must not contain any holes (so, using cp(1) to create the file is not acceptable). |
| 1 | perry | 37 | |
| 38 | |||
| 39 | !!OPTIONS | ||
| 40 | |||
| 3 | CraigBox | 41 | ;__-c__ : Check the device (if it is a block device) for bad blocks before creating the swap area. If any are found, the count is printed. |
| 42 | ;__-f__ : Force - go ahead even if the command is stupid. This allows the creation of a swap area larger than the file or partition it resides on. On SPARC, force creation of the swap area. Without this option __mkswap__ will refuse to create a v0 swap on a device with a valid SPARC superblock, as that probably means one is going to erase the partition table. | ||
| 43 | ;__-p __ ''PSZ'' : Specify the page size to use. | ||
| 44 | ;__-v0__ : Create an old style swap area. | ||
| 45 | ;__-v1__ : Create a new style swap area. | ||
| 1 | perry | 46 | |
| 47 | If no -v option is given, __mkswap__ will default to new style, but use old style if the current kernel is older than 2.1.117 (and also if PAGE_SIZE is less than 2048). The new style header does not touch the first block, so may be preferable, in case you have a boot loader or disk label there. If you need to use both 2.0 and 2.2 kernels, use the -v0 option when creating the swapspace. | ||
| 48 | |||
| 49 | |||
| 50 | !!SEE ALSO | ||
| 3 | CraigBox | 51 | fdisk(8), swapon(8) |
| 1 | perry | 52 | |
| 53 | ---- | ||
| 2 | DanFurlani | 54 | |
| 6 | JohnMcPherson | 55 | __After I create a swapfile (with dd), call mkswap on it, call swapon, do I need to edit /etc/fstab so the swapfile will be remounted upon next boot? More details than just "in some boot script" would be nice.__ |
| 56 | |||
| 57 | It's distro-specific; for example on DebianLinux, there is an /etc/init.d/checkroot.sh and mountall.sh which run swapon(8) after mounting swap partitions from the fstab(5) file. So, just put it in /etc/fstab as type ''swap'', and your distro should deal with it automatically. Similarly, [Slackware] does ''swapon -a'' in its /etc/rc.d/rc.S startup script. According to the swapon(8) manpage, "swapon -a" mounts swap devices listed in /etc/fstab. Perhaps swap __files__ (rather than swap __partitions__) need to be handled differently? |
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