Ceph

Description of problem:
filestore_xfs_extsize defaults to "true" in Hammer. This option is designed to reduce fragmentation on the OSDs.

Tests have found that disabling filestore_xfs_extsize in Hammer improves large sequential write performance by about 20% (Ben Turner's cluster), and in some other tests by lesser amounts. This brings us closer to the large sequential write performance of Firefly.

1. Leave filestore_xfs_extsize unset (currently defaults to "true"),
2. Run large sequential writes tests via CBT, note performance numbers,
3. Set filestore_xfs_extsize to "false",
4. Re-run large sequential writes in CBT, note performance numbers.

Actual results:
The default filestore_xfs_extsize setting results in a large sequential write performance degradation.

Expected results:
The default filestore_xfs_extsize setting should not result in a large sequential write performance degradation.

The proposed fix is to set filestore_xfs_extsize back to "false" in src/common/config_opts.h.

Additional info:
This change comes with a trade-off, because it introduces fragmentation on the OSDs. To address this, we should introduce documentation which explains the fragmentation cost and suggest that users who have a large sequential read use-cases (object storage/CDN) toggle the value to reduce the fragmentation impact over time.