Re: [patch] Add support for barriers to blk{back,front} drivers.

[Gerd Hoffmann <kraxel@suse.de>]

Keir Fraser wrote:
> On 9/11/06 12:48, "Gerd Hoffmann" <kraxel@suse.de> wrote:
> 
>>> What happens (even on native Linux) if you have, say, a RAID array in which
>>> some of the discs support barriers and others don't?
>> The raid0 driver doesn't support barriers in the first place.  Not sure
>> about the other raid drivers.
> 
> Does this mean journalling filesystems cannot run reliably on top of RAID-0?
> That sounds a bit concerning!

Without barrier support available linux filesytems fallback to just wait
until the requests (which would have been submitted as barrier requests)
are finished before submitting the next to make sure the ordering is
fine.  That works ok as long as the disk doesn't do write caching, so
you better turn write caching off in that case.

> What about when running on top of LVM, where an LV is stitched together from
> bits of various PVs. Some may support barriers, some may not. Running on top
> of LVM is default for most distros, so surely it must have a story on write
> barriers?

lvm works ok for me, almost all my domU disks are on lvm.  I have just
one disk in the system though.

I'm not sure what happens with multiple PVs.  I'd expect barriers are
working fine as long as your logical volume is not spread over multiple
physical volumes and the underlying physical volume can handle barriers.

cheers,

  Gerd

-- 
Gerd Hoffmann <kraxel@suse.de>
http://www.suse.de/~kraxel/julika-dora.jpeg