how to know ext cache hit rate?

[Theodore Ts'o]

On Wed, Feb 26, 2014 at 04:01:00PM +0800, fsluck wrote:
> i think it is page cache.  my case is in virualization environment,
> because vm's disk image is much larger than system's cache, i want
> to know when many vms are running if system cache hit rate is very
> low?  if there is a need to seek for other way to suit
> virtualization.

Why not simply measure the I/O rate on the disk?  If there are cache
misses, they will show up as I/O requests, and it's the cache misses
that you care about.  Or more to the point, what's important is
determining whether I/O is the bottleneck or whether something else
like memory pressure.

Also note that with Linux, any available memory will be used for
caching.  If the cache is very small, it's likely the real problem is
that your system doesn't have enough memory --- which is very often
the problem with virtualization environments, and also why Amazon
charges $$$$$ for VM's that have a large amount of memory.

     	 					- Ted