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Re: [dm-devel] [PATCHSET block#for-2.6.36-post] block: replace barrier with sequenced flush



Hello, Christoph.

On 08/13/2010 01:48 PM, Christoph Hellwig wrote:
> The patchset looks functionally correct to me, and with a small patch
> to make use of WRITE_FUA_FLUSH survives xfstests, and instrumenting the
> underlying qemu shows that we actually get the flush requests where we should.

Great.

> No performance or power fail testing done yet.
>
> But I do not like the transition very much.  The new WRITE_FUA_FLUSH
> request is exactly what filesystems expect from a current barrier
> request, so I'd rather move to that functionality without breaking stuff
> inbetween.
> 
> So if it was to me I'd keep patches 1, 2, 4 and 5 from your series, than
> a main one to relax barrier semantics, then have the renaming patches 7
> and 8, and possible keep patch 11 separate from the main implementation
> change, and if absolutely also a separate one to introduce REQ_FUA and
> REQ_FLUSH in the bio interface, but keep things working while doing
> this.

There are two reason to avoid changing the meaning of REQ_HARDBARRIER
and just deprecate it.  One is to avoid breaking filesystems'
expectations underneath it.  Please note that there are out-of-tree
filesystems too.  I think it would be too dangerous to relax
REQ_HARDBARRIER.

Another is that pseudo block layer drivers (loop, virtio_blk,
md/dm...) have assumptions about REQ_HARDBARRIER behavior and things
would be broken in obscure ways between REQ_HARDBARRIER semantics
change and updates to each of those drivers, so I don't really think
changing the semantics while the mechanism is online is a good idea.

> Then we can patches do disable the reiserfs barrier "optimization" as
> the very first one, and DM/MD support which I'm currently working on
> as the last one and we can start doing the heavy testing.

Oops, I've already converted loop, virtio_blk/lguest and am working on
md/dm right now too.  I'm almost done with md and now doing dm. :-)
Maybe we should post them right now so that we don't waste too much
time trying to solve the same problems?

Thanks.

-- 
tejun


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