definitions for /proc/fs/xfs/stat

[Stefan Ring]

> I asked around and people believe the cache is on the order of a few GB and
> the test ran I was intentionally large enough to overshadow any cache
> effects, running for about a minute and doing 100K 1K file creates.  The
> disk write data was close to a sustained 475MB/sec and would have easily
> filled the cache in the first handful of seconds which would have produced
> enough backpressure to slow down the write rate which it never did.

If a limited number of blocks gets written over and over, you won't
ever fill the cache.