Linux: echo 3 > /proc/sys/vm/drop_caches takes hours to complete

Question

I have a Thecus N8900 NAS, which is a Linux based file server, providing files via NFS to six clients. For some reason that Thecus support has yet to explain, it runs a script that checks /proc/meminfo every 60 seconds and if the disk cache exceeds 50% of available RAM they do a "echo 3 > /proc/sys/vm/drop_caches" command to flush the cache.

Leaving aside the issue of whether that makes sense or not, the actual "echo 3 > /proc/sys/vm/drop_caches" command can take hours to complete, which seems way too long to me.

The big problem is that when this happens, the load on the machine spikes, as does the disk utilization, making all NFS traffic crawl until the command finally completes, at which point things are responsive again.

The NAS itself has 16 gigs of RAM, 7 drives in a raid6 configuration (plus a hot spare), no drive problems at all (according to S.M.A.R.T. tests).

So the question is: what would cause the drop_caches command to take so long?

Answer 1

The command itself should complete instantaneously. The consequences, i.e. everything needs to be cached again, can take a lot of time. It doesn't make sense: if you can remove it completely it would be a good idea. (Also this is off topic in StackOverflow)

Edit: does it executes also a sync before echo 3 > /proc/sys/vm/drop_caches, such in sync; echo 3 > /proc/sys/vm/drop_caches? Because the sync operation, which flushes all writes to the disk, may take a bit to complete. Also, while also the sync have performance issue, it may have some sense, in case of sudden power failure the data has been written to the disk already so you are going to be safe.