Re: scsi vs ide performance on fsync's

Jens Axboe
2001-03-07T14:13:53Z

On Wed, Mar 07 2001, Stephen C. Tweedie wrote:
> > SCSI has ordered tag, which fit the model Alan described quite nicely.
> > I've been meaning to implement this for some time, it would be handy
> > for journalled fs to use such a barrier. Since ATA doesn't do queueing
> > (at least not in current Linux), a synchronize cache is probably the
> > only way to go there.
>
> Note that you also have to preserve the position of the barrier in the
> elevator queue, and you need to prevent LVM and soft raid from
> violating the barrier if different commands end up being sent to
> different disks.

Yep, it's much harder than it seems. Especially because for the barrier
to be really useful, having inter-request dependencies becomes a
requirement. So you can say something like 'flush X and Y, but don't
flush Y before X is done'.

--
Jens Axboe