[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: NRic Wheeler <rwheeler@redhat.com> Cc: <stable@vger.kernel.org> Signed-off-by: NJames Bottomley <JBottomley@Parallels.com>
Showing
想要评论请 注册 或 登录