Re: Weird XFS WAL problem

Matthew Wakeling <matthew@flymine.org> wrote:
> On Thu, 3 Jun 2010, Craig James wrote:
>>> Also, are barriers *on* on the RAID1 mount and off on the RAID10
one?
>>
>> It was the barriers.  "barrier=1" isn't just a bad idea on ext4,
>> it's a disaster.
>
> This worries me a little. Does your array have a battery-backed
> cache? If so, then it should be fast regardless of barriers
> (although barriers may make a small difference). If it does not,
> then it is likely that the fast speed you are seeing with barriers
> off is unsafe.

I've seen this, too (with xfs).  Our RAID controller, in spite of
having BBU cache configured for writeback, waits for actual
persistence on disk for write barriers (unlike for fsync).  This
does strike me as surprising to the point of bordering on qualifying
as a bug.  It means that you can't take advantage of the BBU cache
and get the benefit of write barriers in OS cache behavior.  :-(

-Kevin