tree 815334dee3eabcf0cc35cd23928cc456c0212d2d
parent 09c9d4c9b6a2b5909ae3c6265e4cd3820b636863
author Mikulas Patocka <mpatocka@redhat.com> 1294948787 +0000
committer Alasdair G Kergon <agk@redhat.com> 1294948787 +0000

dm io: remove BIO_RW_SYNCIO flag from kcopyd

Remove the REQ_SYNC flag to improve write throughput when writing
to the origin with a snapshot on the same device (using the CFQ I/O
scheduler).

Sequential write throughput (chunksize of 4k, 32k, 512k)
  unpatched:  8.5,  8.6,  9.3 MB/s
  patched:   15.2, 18.5, 17.5 MB/s

Snapshot exception reallocations are triggered by writes that are
usually async, so mark the associated dm_io_request as async as well.
This helps when using the CFQ I/O scheduler because it has separate
queues for sync and async I/O.  Async is optimized for throughput; sync
for latency.  With this change we're consciously favoring throughput over
latency.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>