Re: [PATCH] btrfs: always write superblocks synchronously

[Chris Mason]

On 05/03/2017 04:36 AM, Jan Kara wrote:
> On Tue 02-05-17 09:28:13, Davidlohr Bueso wrote:
> > Commit b685d3d65ac7 "block: treat REQ_FUA and REQ_PREFLUSH as
> > synchronous" removed REQ_SYNC flag from WRITE_FUA implementation.
> > Since REQ_FUA and REQ_FLUSH flags are stripped from submitted IO
> > when the disk doesn't have volatile write cache and thus effectively
> > make the write async. This was seen to cause performance hits up
> > to 90% regression in disk IO related benchmarks such as reaim and
> > dbench[1].
> >
> > Fix the problem by making sure the first superblock write is also
> > treated as synchronous since they can block progress of the
> > journalling (commit, log syncs) machinery and thus the whole filesystem.

> > Fixes: b685d3d65ac (block: treat REQ_FUA and REQ_PREFLUSH as synchronous)
> > Cc: stable <stable@xxxxxxxxxxxxxxx>
> > Cc: Jan Kara <jack@xxxxxxx>
> > Signed-off-by: Davidlohr Bueso <dbueso@xxxxxxx>
>
> I wasn't patient enough and already sent the fix as part of my series
> fixing other filesystems [1]. It also fixes one more place in btrfs that
> needs REQ_SYNC to return to the original behavior.


Thanks guys.

-chris