Is running sync before drop_caches necessary?
caching, linux-kernel, performance
Solution
Writing to `drop_caches` frees only clean caches.
Executing `sync` writes back changed data, i.e., converts dirty caches into clean caches.
Problem
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, drop_cache is a non-destructive operation, 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".