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SubjectMD/DM and barriers (was Re: [patch] ext2/3: document conditions when reliable operation is possible)
On 08/27/2009 09:10 AM, Ric Wheeler wrote:
> One thing that does need fixing for some MD configurations is to stress
> again that we need to make sure that barrier operations are properly
> supported or users will need to disable the write cache on devices with
> volatile write caches.

Agreed; chime in on Christoph's linux-vfs thread if people have input.

I quickly glanced at MD and DM. Currently, upstream, we see a lot of

if (unlikely(bio_barrier(bio))) {
bio_endio(bio, -EOPNOTSUPP);
return 0;
}

in DM and MD make_request functions.

Only md/raid1 supports barriers at present, it seems. None of the other
MD drivers support barriers.

DM has some barrier code... but the above code was pasted from DM's
make_request function, so I am guessing that DM's barrier stuff is
incomplete and disabled at present.

I've been mentioning this issue for years... glad some people finally
noticed :)

Jeff





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