Tag Archives: /dev/shm

Oracle Grid 12c and Linux 7: PRVE-0421 : No entry exists in /etc/fstab for mounting /dev/shm

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When you run Oracle Grid 12c ‘cluvfy stage -pre crsinst’ on Linux 7 (Oracle Enterprise Linux 7.2 in my case) you will get the following error:

Starting check for /dev/shm mounted as temporary file system ...

ERROR:

PRVE-0421 : No entry exists in /etc/fstab for mounting /dev/shm
PRVE-0421 : No entry exists in /etc/fstab for mounting /dev/shm
PRVE-0421 : No entry exists in /etc/fstab for mounting /dev/shm
PRVE-0421 : No entry exists in /etc/fstab for mounting /dev/shm

Check for /dev/shm mounted as temporary file system failed

When you do a ‘df -h’ you will see that ‘/dev/shm’ actually is mounted:

Filesystem Size Used Avail Use% Mounted on
devtmpfs 24G 0 24G 0% /dev
tmpfs    24G 0 24G 0% /dev/shm

This is due to a bug (Doc ID 2065603.1) which comes from the case that the cluvfy check does not check the actual mount of the file system, but looks for the persistence line of this mount in ‘/etc/fstab’ like it did in Linux 6. In Linux 7 you get this tmpfs mount by default.

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Bug: /def/shm does not use /etc/fstab attributes after reboot

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SOLVED: this issue is solved in Linux 6.4 (kernel: 2.6.39-400.17.1.el6uek and 2.6.32-358.el6).
A single entry in /etc/fstab like [tmpfs /dev/shm tmpfs size=3g 0 0] now works as it should!

There is a bug in Red Hat Linux 6 and Oracle Enterprise Linux 6 (UEK and RHEL-kernel) and probably all other Red Hat 6 related Linux Distro’s.

When you need more memory for SGA/PGA when using MEMORY_MAX_TARGET, you need to resize /dev/shm. By default this is 50% of total memory and Oracle tells you to add the following to /etc/fstab, ‘mounting’ the /dev/shm twice (?):

tmpfs /dev/shm tmpfs size=3g 0 0

Which works…

IMPORTANT NOTE: make sure the first field (fs_spec) ‘shmfs’ or ‘tmpfs’ gets the same name as the already existing ‘defaults’ name. So if you have a line [tmpfs /dev/shm tmpfs defaults 0 0], make sure the ‘overruled’ line also starts with ‘tmpfs’: [tmpfs /dev/shm tmpfs size=3g 0 0].

If not, a `mount -a` will un-mount (!!!) the ‘shmfs’ and remount ‘tmpfs’, this results in immediate clearing the ‘/dev/shm’ memory and all your SGA is instantly gone! Running this when databases are running, your databases with AMM will crash! This ‘issue’ is still there last time checked in Linux 7.3. In Oracle documentation about /dev/shm, the first field is ‘shmfs’ with can result in crashing databases when a `mount -a` done!

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