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

Looks like it's a bit harder to disable mbcache than I thought. I ended up adding code to collect the statics.

With selinux enabled, for new_fserver workload of aim7, there are a total of 0x7e05420100000000 ext4_xattr_cache_find() calls that result in a hit and 0xc100000000000000 calls that are not.  The number does not seem to favor the complete disabling of mbcache in this case.

Thanks,
Mak.