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

From: Jeff Garzik
Date: Thu Aug 27 2009 - 12:54:53 EST

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 :)


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