Re: Device loses barrier support (was: Fixed patch for simple barriers.)

From: Andi Kleen
Date: Thu Dec 04 2008 - 20:26:43 EST

> * barrier support in md-raid1 deviates from the specification at
> Documentation/block/barrier.txt. The specification says that requests
> submitted after the barrier request hit the media after the barrier
> request hits the media. The reality is that the barrier request can be
> randomly aborted and the requests submitted after it hit the media before
> the barrier request.

Yes the spec should be probably updated.

But also see Linus' rant from yesterday about code vs documentation.
When in doubt the code wins.
> * the filesystems developed hacks to work around this issue, the hacks
> involve not submitting more requests after the barrier request,

I suspect the reason the file systems did it this way is that
it was a much simpler change than to rewrite the transaction
manager for this.

> synchronously waiting for the barrier request and eventually retrying it.
> These hacks suppress any performance advantage barriers could bring.
> * you submit a patch that makes barriers even more often deviate from the
> specification and you argue that the patch is correct because filesystems
> handle this deviation.

Sorry what counts is the code behaviour, not the specification.


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