linux-kernel - Re: [PATCH 1/3] nbd: support FLUSH requests

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Message-Id: <115932E2-48BE-406E-9E75-61D12EB5937A@alex.org.uk>
Date:	Tue, 12 Feb 2013 17:37:04 +0000
From:	Alex Bligh <alex@...x.org.uk>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	Alex Bligh <alex@...x.org.uk>, linux-kernel@...r.kernel.org,
	<nbd-general@...ts.sf.net>,
	Paul Clements <Paul.Clements@...eleye.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 1/3] nbd: support FLUSH requests

Paolo,

> Add support for a new flag that the server can pass.  If the flag is
> enabled, we translate REQ_FLUSH requests into the NBD_CMD_FLUSH
> command.
> 
> Cc: <nbd-general@...ts.sf.net>
> Cc: Paul Clements <Paul.Clements@...eleye.com>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Signed-off-by: Alex Bligh <alex@...x.org.uk>
> [ Removed FUA support for reasons similar to those outlined in
>  https://lkml.org/lkml/2010/8/17/234 for virtio - Paolo ]
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>


For my education, why remove the FUA stuff? The link you posted
was for virtio drivers, and says:

> > Hmmm... the underlying storage could be md/dm RAIDs in which case FUA
> > should be cheaper than FLUSH.
> 
> If someone ever wrote a virtio-blk backend that sits directly ontop
> of the Linux block layer that would be true.

In this case we don't know what the backend is sitting on top of
a-priori. It might be the current nbd server code, but it might
not be.

-- 
Alex Bligh




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