MD/DM and barriers (was Re: [patch] ext2/3: document conditions whenreliable operation is possible)

[Jeff Garzik]

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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