diff options
| author | James Bottomley <JBottomley@Parallels.com> | 2013-04-24 14:02:53 -0700 |
|---|---|---|
| committer | James Bottomley <JBottomley@Parallels.com> | 2013-05-02 16:15:54 -0700 |
| commit | 39c60a0948cc06139e2fbfe084f83cb7e7deae3b (patch) | |
| tree | 7d8b3f42a70e8f0caea47bebc1b113f79753baca /drivers/scsi/sd.h | |
| parent | 14b06808d2848658517657ae4c97445da1e14890 (diff) | |
[SCSI] sd: fix array cache flushing bug causing performance problems
Some arrays synchronize their full non volatile cache when the sd driver sends
a SYNCHRONIZE CACHE command. Unfortunately, they can have Terrabytes of this
and we send a SYNCHRONIZE CACHE for every barrier if an array reports it has a
writeback cache. This leads to massive slowdowns on journalled filesystems.
The fix is to allow userspace to turn off the writeback cache setting as a
temporary measure (i.e. without doing the MODE SELECT to write it back to the
device), so even though the device reported it has a writeback cache, the
user, knowing that the cache is non volatile and all they care about is
filesystem correctness, can turn that bit off in the kernel and avoid the
performance ruinous (and safety irrelevant) SYNCHRONIZE CACHE commands.
The way you do this is add a 'temporary' prefix when performing the usual
cache setting operations, so
echo temporary write through > /sys/class/scsi_disk/<disk>/cache_type
Reported-by: Ric Wheeler <rwheeler@redhat.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>
Diffstat (limited to 'drivers/scsi/sd.h')
| -rw-r--r-- | drivers/scsi/sd.h | 1 |
1 files changed, 1 insertions, 0 deletions
diff --git a/drivers/scsi/sd.h b/drivers/scsi/sd.h index 74a1e4ca540..2386aeb41fe 100644 --- a/drivers/scsi/sd.h +++ b/drivers/scsi/sd.h @@ -73,6 +73,7 @@ struct scsi_disk { u8 protection_type;/* Data Integrity Field */ u8 provisioning_mode; unsigned ATO : 1; /* state of disk ATO bit */ + unsigned cache_override : 1; /* temp override of WCE,RCD */ unsigned WCE : 1; /* state of disk WCE bit */ unsigned RCD : 1; /* state of disk RCD bit, unused */ unsigned DPOFUA : 1; /* state of disk DPOFUA bit */ |