关于/dev/shm的文章

https://wiki.archlinux.org/index.php/Tmpfs
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tmpfs

tmpfs is
a temporary filesystem that resides in memory and/or your swap partition(s), depending on how much you fill it up. Mounting directories as tmpfs can be an effective way of speeding up accesses to their files, or to ensure that their contents are automatically
cleared upon reboot.

Note: When using systemd, temporary files in tmpfs
directories can be recreated at boot by using tmpfiles.d.

Contents

 [hide] 
1 Usage
2 Examples
3 Disable
automatic mount
4 Troubleshooting
4.1 Opening
symlinks in tmpfs as root fails

5 See
also

Usage

Some directories where tmpfs is commonly used are /tmp, /var/lock and /var/run.
Do not use it on/var/tmp,
because that folder is meant for temporary files that are preserved across reboots.
Arch uses a tmpfs 

/run

 directory, with 

/var/run

 and 

/var/lock

 simply
existing as symlinks for compatibility. It is also used for 

/tmp

 by the default systemd setup and does not require an entry in fstab unless
a specific configuration is needed.
glibc 2.2
and above expects tmpfs to be mounted at 

/dev/shm

 for POSIX
shared memory. Mounting tmpfs at 

/dev/shm

is handled automatically by systemd,
so manual configuration in fstab is no longer necessary.
Generally, I/O intensive tasks and programs that run frequent read/write operations can benefit from using a tmpfs folder. Some applications can even receive a substantial gain by offloading some (or all) of
their data onto the shared memory. For example, relocating
the Firefox profile into RAM shows a significant improvement in performance.

Examples

By default, a tmpfs partition has its maximum size set to half your total RAM, but this can be customized. Note that the actual memory/swap consumption depends on how much you fill it up, as tmpfs partitions
do not consume any memory until it is actually needed.
To explicitly set a maximum size, in this example to override the default 

/tmp

 mount,
use the 

size

 mount option:

/etc/fstab

tmpfs   /tmp         tmpfs   nodev,nosuid,size=2G          0  0

Here is a more advanced example showing how to add tmpfs mounts for users. This is useful for websites, mysql tmp files, 

~/.vim/

,
and more. It's important to try and get the ideal mount options for what you are trying to accomplish. The goal is to have as secure settings as possible to prevent abuse. Limiting the size, and specifying uid and gid + mode is very secure. For more information
on this subject, follow the links listed in the #See alsosection.

/etc/fstab

tmpfs   /www/cache    tmpfs  rw,size=1G,nr_inodes=5k,noexec,nodev,nosuid,uid=648,gid=648,mode=1700   0  0

See the 

mount

 command man page for more information. One useful mount option
in the man page is the 

default

 option. At least understand that.
Reboot for the changes to take effect. Note that although it may be tempting to simply run 

mount
-a

 to make the changes effective immediately, this will make any files currently residing in these directories inaccessible (this is especially problematic for running programs with lockfiles, for example). However, if all of them are empty, it should
be safe to run 

mount -a

 instead of rebooting (or mount them individually).
After applying changes, you may want to verify that they took effect by looking at 

/proc/mounts

 and
using 

findmnt

:

$ findmnt --target /tmp

TARGET SOURCE FSTYPE OPTIONS
/tmp   tmpfs  tmpfs  rw,nosuid,nodev,relatime

The tmpfs can also be temporarily resized without the need to reboot, for example when a large compile job needs to run soon. In this case, you can run:

# mount -o remount,size=4G,noatime /tmp

Disable automatic mount

Under systemd, 

/tmp

 may
be automatically mounted as a tmpfs even though you have no entry for that in your 

/etc/fstab

.
To disable the automatic mount, run:

# systemctl mask tmp.mount

Files will no longer be stored in a tmpfs, but your block device instead. The 

/tmp

 contents
will now be preserved between reboots, which you might not want. To regain the previous behavior and clean the 

/tmp

 folder automatically
when restarting your machine, consider using 

tmpfiles.d(5)

:

/etc/tmpfiles.d/tmp.conf

# see tmpfiles.d(5)
# always enable /tmp folder cleaning
D! /tmp 1777 root root 0

# remove files in /var/tmp older than 10 days
D /var/tmp 1777 root root 10d

# namespace mountpoints (PrivateTmp=yes) are excluded from removal
x /tmp/systemd-private-*
x /var/tmp/systemd-private-*
X /tmp/systemd-private-*/tmp
X /var/tmp/systemd-private-*/tmp

Troubleshooting

Opening symlinks in tmpfs as root fails

Considering 

/tmp

 is using tmpfs, change the current directory to 

/tmp

,
then create a file and create a symlink to that file in the same 

/tmp

 directory. If you try to open the file you created via
the symlink, you will get a permission denied error. This is expected as 

/tmp

 has
the sticky bit set.
This behaviour can be controlled via 

/proc/sys/fs/protected_symlinks

 or simply
via sysctl: 

sysctl -w fs.protected_symlinks=0

. See Sysctl#Configuration to
make this permanent.

Warning: Changing this behaviour can lead to security issues! Disable it only if you know what you are doing.

See also

Linux
kernel documentation