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I have read a lot of docs saying, it is good idea to do a sync before doing echo [1,2,3] > /proc/sys/vm/drop_caches. I am not able to understand why it is needed, since drop_cache is a non-destructive operation, and dirty data is not going to be deleted by drop_cache. I have also seen a behavior where echo 1 > /proc/sys/vm/drop_caches first commit dirty data back to disk and then frees the cache. This is seen through /proc/meminfo "Dirty" & "Writeback".

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    "I have read a lot of docs saying ..." Could you link to such a doc? – Mat Jul 6 '13 at 10:59
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    Sure, Linux Kernel Documentation about "vm" subsystem kernel.org/doc/Documentation/sysctl/vm.txt Excerpt - drop_caches Writing to this will cause the kernel to drop clean caches, dentries and inodes from memory, causing that memory to become free. To free pagecache: echo 1 > /proc/sys/vm/drop_caches To free dentries and inodes: echo 2 > /proc/sys/vm/drop_caches To free pagecache, dentries and inodes: echo 3 > /proc/sys/vm/drop_caches As this is a non-destructive operation and dirty objects are not freeable, the user should run `sync' first. – Subodh Pachghare Jul 8 '13 at 5:54
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Anyways I got the answer on stackoverflow which I corroborated by doing a small experiment.

"sync" only makes dirty cache to clean cache. cache is still preserved. drop_caches doesn't touch dirty caches and only drops clean caches. So to make all memory free, it is necessary to do sync first before drop_caches in case flushing daemons hasn't written the changes to disk.

My blog about this little experiment -

What are exactly O_DIRECT, O_SYNC Flags, Buffers & Cached in Linux-Storage I/O?

Stackoverflow link -

“sync” before drop_caches,is it necessary?

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