On 09/04/2013 08:00 PM, Andreas Dilger wrote:

In the past, I've raised the question of whether mbcache is even
useful on real-world systems.  Essentially, this is providing a
"deduplication" service for ext2/3/4 xattr blocks that are identical.
The question is how often this is actually the case in modern use?
The original design was for allowing external ACL blocks to be
shared between inodes, at a time when ACLs where pretty much the
only xattrs stored on inodes.

The question now is whether there are common uses where all of the
xattrs stored on multiple inodes are identical?  If that is not the
case, mbcache is just adding overhead and should just be disabled
entirely instead of just adding less overhead.

There aren't good statistics on the hit rate for mbcache, but it
might be possible to generate some with systemtap or similar to
see how often ext4_xattr_cache_find() returns NULL vs. non-NULL.

Cheers, Andreas

Thanks Andreas for the comments.  Since I'm not familiar with systemtap, I'm thinking probably the quickest and simplest way is to re-run aim7 and swing bench with mbcache disabled for comparison. Please let me know if you have any other benchmark suggestion or if you think systemtap on ext4_xattr_cache_find() would give a more accurate measurement.

Thanks,
Mak.