Difference between revisions of "Swap"
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− | [[Category: | + | [[Category:File systems]] |
− | + | [[fr:Swap]] | |
+ | [[it:Swap]] | ||
+ | [[pt:Swap]] | ||
+ | [[zh-CN:Swap]] | ||
{{Article summary start}} | {{Article summary start}} | ||
{{Article summary text|An introduction to swap space and paging on GNU/Linux. Covers creation and activation of swap partitions and swap files.}} | {{Article summary text|An introduction to swap space and paging on GNU/Linux. Covers creation and activation of swap partitions and swap files.}} | ||
{{Article summary heading|Related}} | {{Article summary heading|Related}} | ||
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{{Article summary wiki|fstab}} | {{Article summary wiki|fstab}} | ||
{{Article summary end}} | {{Article summary end}} | ||
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:''Linux divides its physical RAM (random access memory) into chucks of memory called pages. Swapping is the process whereby a page of memory is copied to the preconfigured space on the hard disk, called swap space, to free up that page of memory. The combined sizes of the physical memory and the swap space is the amount of virtual memory available.'' | :''Linux divides its physical RAM (random access memory) into chucks of memory called pages. Swapping is the process whereby a page of memory is copied to the preconfigured space on the hard disk, called swap space, to free up that page of memory. The combined sizes of the physical memory and the swap space is the amount of virtual memory available.'' | ||
+ | == Swap space == | ||
Swap space will usually be a disk partition but can also be a file. Users may create a swap space during installation of Arch Linux or at any later time should it become necessary. Swap space is generally recommended for users with less than 1 GB of RAM, but becomes more a matter of personal preference on systems with gratuitous amounts of physical RAM (though it is required for suspend-to-disk support). | Swap space will usually be a disk partition but can also be a file. Users may create a swap space during installation of Arch Linux or at any later time should it become necessary. Swap space is generally recommended for users with less than 1 GB of RAM, but becomes more a matter of personal preference on systems with gratuitous amounts of physical RAM (though it is required for suspend-to-disk support). | ||
+ | |||
+ | To check swap status, use: | ||
+ | $ swapon -s | ||
+ | |||
+ | Or: | ||
+ | $ free -m | ||
+ | |||
+ | {{Note|There is no performance advantage to either a contiguous swap file or a partition, both are treated the same way.}} | ||
+ | |||
== Swap partition == | == Swap partition == | ||
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To enable this swap partition on boot, add an entry to [[fstab]]: | To enable this swap partition on boot, add an entry to [[fstab]]: | ||
/dev/sda2 none swap defaults 0 0 | /dev/sda2 none swap defaults 0 0 | ||
+ | |||
+ | {{Note|If using a TRIM supported SSD, discard is a valid mount option for swap. If creating swap manually, using -d or --discard achieves the same. For more information and other available mount options, see the swapon man page.}} | ||
== Swap file == | == Swap file == | ||
− | As an alternative to creating an entire partition, a swap file offers the ability to vary its size on-the-fly, and is more easily removed altogether. This may be especially desirable if disk space is at a premium (e.g. a modestly-sized SSD). | + | As an alternative to creating an entire partition, a swap file offers the ability to vary its size on-the-fly, and is more easily removed altogether. This may be especially desirable if disk space is at a premium (e.g. a modestly-sized SSD). |
+ | |||
+ | {{Note|The BTRFS filesystem does not currently support swapfiles.}} | ||
=== Swap file creation === | === Swap file creation === | ||
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# fallocate -l 512M /swapfile | # fallocate -l 512M /swapfile | ||
+ | Or | ||
# dd if=/dev/zero of=/swapfile bs=1M count=512 | # dd if=/dev/zero of=/swapfile bs=1M count=512 | ||
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# swapon /swapfile | # swapon /swapfile | ||
− | Edit {{ | + | Edit {{ic|/etc/fstab}} and add an entry for the swap file: |
/swapfile none swap defaults 0 0 | /swapfile none swap defaults 0 0 | ||
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=== Swap file resuming === | === Swap file resuming === | ||
− | Resuming the system from a swap file after hibernation requires an addition parameter | + | Resuming the system from a swap file after hibernation requires an addition [[kernel parameters|kernel parameter]] compared to resuming from a swap partition. The additional parameter is {{ic|1=resume_offset=<Swap File Offset>}}. |
− | The value of | + | The value of {{ic|<Swap File Offset>}} can be obtained from the output of {{ic|filefrag -v}}; The output is in a table format; the required value is located in the {{ic|physical}} column from the first row. Eg: |
# filefrag -v /swapfile | # filefrag -v /swapfile | ||
Filesystem type is: ef53 | Filesystem type is: ef53 | ||
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... | ... | ||
− | From the example | + | From the example {{ic|<Swap FIle Offset>}} is {{ic|7546880}}. |
+ | {{Note|Please note that in kernel {{ic|resume}} parameter you still have to type path to partition (e.g. {{ic|1=resume=/dev/sda1}}) not to swapfile explicitly! Parameter {{ic|resume_offset}} is for informing system where swapfile starts on hard disk (e.g. {{ic|1=resume_offset=7546880}}).}} | ||
== Swap with USB device == | == Swap with USB device == | ||
− | Thanks to modularity offered by Linux, we can have multiple swap partitions spread over different devices. | + | Thanks to modularity offered by Linux, we can have multiple swap partitions spread over different devices. If you have a very full hard disk, USB device can be used as partition temporally. But this method has some severe disadvantage: |
+ | * USB device is slower than hard disk. | ||
+ | * flash memories have limited write cycles. Using it as swap partition will kill it quickly. | ||
+ | * when another device is attached to the computer, no swap can be used. | ||
− | + | To add a a USB device to SWAP, first take a USB flash and partition it with a swap partition.You can use graphical tools such as Gparted or console tools like fdisk. Make sure to label the partition as SWAP before writing the partition table. | |
{{Box RED||Make sure you are writing the partition to the correct disk!}} | {{Box RED||Make sure you are writing the partition to the correct disk!}} | ||
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in the ''original'' swap entry for teaching fstab to use HD swap only when USB is full | in the ''original'' swap entry for teaching fstab to use HD swap only when USB is full | ||
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This guide will work for other memory such as SD cards, etc. | This guide will work for other memory such as SD cards, etc. | ||
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== Performance Tuning == | == Performance Tuning == | ||
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The ''swappiness'' [[sysctl]] parameter represents the kernel's preference (or avoidance) of swap space. Swappiness can have a value between 0 and 100. Setting this parameter to a low value will reduce swapping from RAM, and is known to improve responsiveness on many systems. | The ''swappiness'' [[sysctl]] parameter represents the kernel's preference (or avoidance) of swap space. Swappiness can have a value between 0 and 100. Setting this parameter to a low value will reduce swapping from RAM, and is known to improve responsiveness on many systems. | ||
− | {{ | + | {{hc|/etc/sysctl.conf |
− | | | + | |2=<nowiki> |
− | | | ||
vm.swappiness=1 | vm.swappiness=1 | ||
vm.vfs_cache_pressure=50 | vm.vfs_cache_pressure=50 | ||
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=== Priority === | === Priority === | ||
− | If you have more than one swap file or swap partition you should consider assigning a priority value (0 to 32767) for each swap area. The system will use swap areas of higher priority before using swap areas of lower priority. For example, if you have a faster disk ({{ | + | If you have more than one swap file or swap partition you should consider assigning a priority value (0 to 32767) for each swap area. The system will use swap areas of higher priority before using swap areas of lower priority. For example, if you have a faster disk ({{ic|/dev/sda}}) and a slower disk ({{ic|/dev/sdb}}), assign a higher priority to the swap area located on the faster device. Priorities can be assigned in fstab via the {{Ic|1=pri}} parameter: |
/dev/sda1 none swap defaults,pri=100 0 0 | /dev/sda1 none swap defaults,pri=100 0 0 |
Revision as of 04:15, 9 November 2012
zh-CN:Swap Template:Article summary start Template:Article summary text Template:Article summary heading Template:Article summary wiki Template:Article summary end
From All about Linux swap space:
- Linux divides its physical RAM (random access memory) into chucks of memory called pages. Swapping is the process whereby a page of memory is copied to the preconfigured space on the hard disk, called swap space, to free up that page of memory. The combined sizes of the physical memory and the swap space is the amount of virtual memory available.
Contents
Swap space
Swap space will usually be a disk partition but can also be a file. Users may create a swap space during installation of Arch Linux or at any later time should it become necessary. Swap space is generally recommended for users with less than 1 GB of RAM, but becomes more a matter of personal preference on systems with gratuitous amounts of physical RAM (though it is required for suspend-to-disk support).
To check swap status, use:
$ swapon -s
Or:
$ free -m
Swap partition
A swap partition can be created with most GNU/Linux partitioning tools (e.g. fdisk
, cfdisk
). Swap partitions are designated as type 82.
To set up a Linux swap area, the mkswap
command is used. For example:
# mkswap /dev/sda2
To enable the device for paging:
# swapon /dev/sda2
To enable this swap partition on boot, add an entry to fstab:
/dev/sda2 none swap defaults 0 0
Swap file
As an alternative to creating an entire partition, a swap file offers the ability to vary its size on-the-fly, and is more easily removed altogether. This may be especially desirable if disk space is at a premium (e.g. a modestly-sized SSD).
Swap file creation
As root use fallocate
to create a swap file the size of your choosing (M = Megabytes, G = Gigabytes) (dd
can also be used but will take longer). For example, creating a 512 MB swap file:
# fallocate -l 512M /swapfile Or # dd if=/dev/zero of=/swapfile bs=1M count=512
Set the right permissions (a world-readable swap file is a huge local vulnerability)
# chmod 600 /swapfile
After creating the correctly-sized file, format it to swap:
# mkswap /swapfile
Activate the swapfile:
# swapon /swapfile
Edit /etc/fstab
and add an entry for the swap file:
/swapfile none swap defaults 0 0
Remove swap file
To remove a swap file, the current swap file must be turned off.
As root:
# swapoff -a
Remove swapfile:
# rm -rf /swapfile
Swap file resuming
Resuming the system from a swap file after hibernation requires an addition kernel parameter compared to resuming from a swap partition. The additional parameter is resume_offset=<Swap File Offset>
.
The value of <Swap File Offset>
can be obtained from the output of filefrag -v
; The output is in a table format; the required value is located in the physical
column from the first row. Eg:
# filefrag -v /swapfile Filesystem type is: ef53 File size of /swapfile is 4290772992 (1047552 blocks, blocksize 4096) ext logical physical expected length flags 0 0 7546880 6144 1 6144 7557120 7553023 2048 2 8192 7567360 7559167 2048 ...
From the example <Swap FIle Offset>
is 7546880
.
resume
parameter you still have to type path to partition (e.g. resume=/dev/sda1
) not to swapfile explicitly! Parameter resume_offset
is for informing system where swapfile starts on hard disk (e.g. resume_offset=7546880
).Swap with USB device
Thanks to modularity offered by Linux, we can have multiple swap partitions spread over different devices. If you have a very full hard disk, USB device can be used as partition temporally. But this method has some severe disadvantage:
- USB device is slower than hard disk.
- flash memories have limited write cycles. Using it as swap partition will kill it quickly.
- when another device is attached to the computer, no swap can be used.
To add a a USB device to SWAP, first take a USB flash and partition it with a swap partition.You can use graphical tools such as Gparted or console tools like fdisk. Make sure to label the partition as SWAP before writing the partition table.
Next edit the fstab
# nano /etc/fstab
Now add a new entry, just under the current swap entry, which take the current swap partition over the new USB one
UUID=... none swap defaults,pri=10 0 0
where UUID is taken from the output of the command
ls -l /dev/disk/by-uuid/ | grep /dev/sdc1
Just replace sdc1 with your new USB swap partition. sdb1
Last, add
pri=0
in the original swap entry for teaching fstab to use HD swap only when USB is full
This guide will work for other memory such as SD cards, etc.
Performance Tuning
Swap values can be adjusted to help performance.
Swappiness
The swappiness sysctl parameter represents the kernel's preference (or avoidance) of swap space. Swappiness can have a value between 0 and 100. Setting this parameter to a low value will reduce swapping from RAM, and is known to improve responsiveness on many systems.
/etc/sysctl.conf
vm.swappiness=1 vm.vfs_cache_pressure=50
Priority
If you have more than one swap file or swap partition you should consider assigning a priority value (0 to 32767) for each swap area. The system will use swap areas of higher priority before using swap areas of lower priority. For example, if you have a faster disk (/dev/sda
) and a slower disk (/dev/sdb
), assign a higher priority to the swap area located on the faster device. Priorities can be assigned in fstab via the pri
parameter:
/dev/sda1 none swap defaults,pri=100 0 0 /dev/sdb2 none swap defaults,pri=10 0 0
Or via the −p
(or −−priority
) parameter of swapon:
# swapon -p 100 /dev/sda1
If two or more areas have the same priority, and it is the highest priority available, pages are allocated on a round-robin basis between them.