Increase or Decrease tmpfs partition size in Linux
In my last article I gave an overview on tuned and steps to create a custom tuned profile as per your performance requirement. In this article I will share the steps to (increase/decrease) change tmpfs partitions size in Linux. With RHEL 7 the Linux kernel provides a number of different ways for userspace to communicate with it.
For many facilities there are system calls, others are hidden behind
netlink interfaces, and even others are exposed via virtual file systems
such as /proc or /sys. These file systems are programming
interfaces, they are not actually backed by real, persistent storage.
They simply use the file system interface of the kernel as interface to
various unrelated mechanisms.
Now by default systemd assigns a certain part of your physical memory to these partitions as a threshold. But what if your requirement requires you to change tmpfs partition size?
For some of the tmpfs partitions, you can change the threshold size by
using fstab. While for other partitions like (/run/user/) which are
created runtime, you cannot use fstab to change tmpfs partition size
for such runtime directories.
Below are the list of tmpfs partitions available in RHEL 7
Filesystem Size Used Avail Use% Mounted on
tmpfs 187G 0 187G 0% /dev/shm
tmpfs 187G 41M 187G 1% /run
tmpfs 187G 0 187G 0% /sys/fs/cgroup
tmpfs 38G 0 38G 0% /run/user/1710
tmpfs 38G 0 38G 0% /run/user/0
/etc/fstab does not contains entries for these
tmpfs partitions but still df -h will show these partitions.
Change tmpfs partition size for /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 2GiB, change the following
line in /etc/fstab:
Default:
none /dev/shm tmpfs defaults 0 0
To:
none /dev/shm tmpfs defaults,size=2G 0 0
For the changes to take effect immediately remount /dev/shm:
# mount -o remount /dev/shm
Lastly validate the new size
# df -h /dev/shm
Filesystem Size Used Avail Use% Mounted on
tmpfs 2.0G 0 2.0G 0% /dev/shm
Change tmpfs partition size for /run
/run is a filesystem which is used by applications the same way
/var/run was used in previous versions of RHEL. Now /var/run is a
symlink to /run filesystem. Previously early boot programs used to
place runtime data in /dev under numerous hidden dot directories. The
reason they used directories in /dev was because it was known to be
available from very early time during machine boot process. Because
/var/run was available very late during boot, as /var might reside on
a separate file system, directory /run was implemented.
By default you may not find any /etc/fstab entry for /run, so you can
add below line
none /run tmpfs defaults,size=600M 0 0
For the changes to take effect immediately remount /run:
# mount -o remount /run
lastly validate the new size
# df -h /run
Filesystem Size Used Avail Use% Mounted on
tmpfs 600M 9.6M 591M 2% /run
Change tmpfs partition size for /run/user/$UID
/run/user/$UID is a filesystem used by pam_systemd to store files used by <a title="5 practical examples to list running processes in Linux" href="https://www.golinuxcloud.com/linux-list-processes/" target="_blank" rel="noopener noreferrer">running processes</a> for that user. In previous releases these files were typically stored in <code>/tmp
as it was the only location specified by the FHS which is local, and
writeable by all users. However using /tmp can causes issues because
it is writeable by anyone and thus access control was challenging. Using
/run/user/$UID fixes the issue because it is only accessible by the
target user.
/run/user/$UID using
/etc/fstab.
tmps partition size for /run/user/$UID is taken based on
RuntimeDirectorySize value from /etc/systemd/logind.conf
# grep -i runtime /etc/systemd/logind.conf
RuntimeDirectorySize=10%
By default the default threshold for these runtime directory is 10% of
the total physical memory.
From the man page of logind.conf
RuntimeDirectorySize=
Sets the size limit on the $XDG_RUNTIME_DIR runtime directory for each user who logs in. Takes a size in bytes, optionally suffixed with the usual K, G, M, and T suffixes, to the base 1024 (IEC). Alternatively, a numerical percentage suffixed by "%" may be specified, which sets the size limit relative to the amount of physical RAM. Defaults to 10%. Note that this size is a safety limit only. As each runtime directory is a tmpfs file system, it will only consume as much memory as is needed.
Modify this variable to your required value, for example I have provided
threshold of 100M
# grep -i runtime /etc/systemd/logind.conf
RuntimeDirectorySize=100M
Next restart the systemd-logind service
Change tmpfs partition size for /sys/fs/cgroup
/sys/fs/cgroup is an interface through which Control Groups can be
accessed. By default there may or may not be /etc/fstab content for
/sys/fs/cgroup so add a new entry
Current value for /sys/fs/cgroup
# df -h /sys/fs/cgroup
Filesystem Size Used Avail Use% Mounted on
tmpfs 63G 0 63G 0% /sys/fs/cgroup
Add below line in your /etc/fstab to change the threshold to 2GB
none /sys/fs/cgroup tmpfs defaults,size=2G 0 0
Remount the partition /sys/fs/cgroup
# mount -o remount /sys/fs/cgroup
Lastly validate the updated changes
# df -h /sys/fs/cgroup
Filesystem Size Used Avail Use% Mounted on
tmpfs 2.0G 0 2.0G 0% /sys/fs/cgroup
References:
Why are there many tmpfs filesystems mounted
on the server?
What is the purpose of the /run/user/1000,
tmpfs filesystem that appears in df?
How to work with /dev/shm in Red Hat
Enterprise Linux 7
How do I modify the size of tmpfs?
Lastly I hope the steps from the article to change tmpfs partition size on Linux was helpful. So, let me know your suggestions and feedback using the comment section.
