如何设置tmpfs的大小

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How do I modify the size of tmpfs?

 SOLUTION 已验证 - 已更新 2015年六月18日13:28 - 

English 

环境

• Red Hat Enterprise Linux 7
• Red Hat Enterprise Linux 6
• Red Hat Enterprise Linux 5
• Red Hat Enterprise Linux 4

问题

• How can an in-ram temporary filesystem be created?
• To increase the default size of tmpfs

决议

Modifying the size of tmpfs on /dev/shm

If an application is POSIX compliant or it uses GLIBC (2.2 and above) on a Red Hat Enterprise Linux system, it will usually use the /dev/shm for shared memory (shm_open, shm_unlink). /dev/shm is a temporary filesystem (tmpfs) which is mounted from /etc/fstab. Hence the standard options like "size" supported for tmpfs can be used to increase or decrease the size of tmpfs on /dev/shm (by default it is half of available system RAM).

For example, to set the size of /dev/shm to 1GiB, change the following line in /etc/fstab:

Raw

Default:
none     /dev/shm       tmpfs   defaults        0 0

Changed:
none     /dev/shm       tmpfs   defaults,size=1G        0 0             <<== add "size=1G"

For the changes to take effect immediately remount /dev/shm:

Raw

 # mount -o remount /dev/shm

Creating a new tmpfs mount

• Example of mounting tmpfs on the fly with no options on a system with 4GB of RAM

Raw

# mkdir /mnt/tmp
# mount -t tmpfs tmpfs /mnt/tmp
# df -h /mnt/tmp
Filesystem      Size  Used Avail Use% Mounted on
tmpfs           1.9G     0  1.9G   0% /mnt/tmp

• Example of mounting two tmpfs with specific sizes via fstab

Add the following lines to /etc/fstab:

Raw

tmpfs    /tmp1    tmpfs    size=200m    0 0
tmpfs    /tmp2    tmpfs    size=1g    0 0

Run the following commands:

Raw

# mkdir /tmp{1,2}
# mount -a
# df -h /tmp?
Filesystem            Size  Used Avail Use% Mounted on
tmpfs                 200M     0  200M   0% /tmp1
tmpfs                 1.0G     0  1.0G   0% /tmp2

Additional notes

• tmpfs is mounted solely in memory, allowing the kernel to avoid any disk I/O layers altogether; reads and writes go directly to the page cache. Following from this, all data in a tmpfs is lost when the tmpfs is unmounted.

• By default, tmpfs are created with half of the active RAM size. If this value requires changing, use the optional size= mount option as seen above to set the desired size.

• Note that tmpfs is backed by swap space -- if swap is available, tmpfs pages might be swapped out to disk.

• See the "Mount options for tmpfs" section of the mount(8) manual page (man mount) for a full explanation of the available mount options. Further details regarding tmpfs is available in /usr/share/doc/kernel-doc-<version>/Documentation/filesystems/tmpfs.txt file.

• A mount -o remount to shrink a tmpfs will succeed if there are not any blocks or inodes allocated within the new limit of the smaller tmpfs size. It is not possible to predict or control this, however a remount simply will not work if it cannot be done. In that case, stop all processes using tmpfs, unmount it, and remount it using the new size.

• A mount -o remount to grow a tmpfs will always succeed.

WARNINGS

• The mount command allows the creation of a tmpfs larger than available RAM. This is usually a bad idea, and should absolutely be avoided if system swap is unavailable.
• If no system swap is available, make sure the system has enough RAM to accommodate the size and usage of any tmpfs being created.
• In Red Hat Enterprise Linux 6.0 - 6.3, use of the "size=" mount option for a tmpfs mount in /etc/fstab has no affect at boot time. See tmpfs size option specified in fstab is ignored during boot proccess for more details.

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• mount

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