Linux empty buffer cache command permission denied

Question

I'm using the following commands to flush cache:

# sync; echo 3 > /proc/sys/vm/drop_caches
# sync; echo 2 > /proc/sys/vm/drop_caches
# sync; echo 1 > /proc/sys/vm/drop_caches

When I use one of the above commands, I get the following:

-bash: /proc/sys/vm/drop_caches: Permission denied

My system running CentOS 6.5, I logged in as root via SSH.

Any suggestions?

Solution 1

this one should work regardless the linux distribution you are running. You need to cover the redirection when running sudo.

$ sudo sh -c "$(which echo) 3 > /proc/sys/vm/drop_caches"

Credits: https://unix.stackexchange.com/a/148442/101951 (i added the which echo part though)

Solution 2

I'm not sure why are you're running the 3 commands, when the first one is all you need:

$ sync; echo 3 > /proc/sys/vm/drop_caches

This will do the following 3 things: free pagecache, dentries and inodes. Using 1 only frees the pagecache while 2 frees the dentries + inodes.

You're probably experiencing some weird timing issue where:

1. You're doing this operation too quickly
2. There is nothing to empty since you just previously ran the command
3. Your previous command hasn't had a chance to complete or be processed

References

• How do you empty the buffers and cache on a Linux system?

Comments

[Security Beast]

just for improvement in me may i know y this got depreciated

[jasonwryan]

This adds nothing to the existing answers other than the fact it is not in English...

[heemayl]

Also whats the point of sudo before echo?

[G-Man Says 'Reinstate Monica']

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