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From: Paolo Bonzini <pbonzini@redhat.com>
To: Alex Bligh <alex@alex.org.uk>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	linux-kernel@vger.kernel.org, nbd-general@lists.sf.net,
	Paul Clements <Paul.Clements@steeleye.com>
Subject: Re: [PATCH 1/3] nbd: support FLUSH requests
Date: Wed, 13 Feb 2013 17:02:10 +0100	[thread overview]
Message-ID: <511BB902.3080302@redhat.com> (raw)
In-Reply-To: <3EDEF735-9A67-439E-BA65-089C6AAFD1BF@alex.org.uk>

Il 13/02/2013 16:55, Alex Bligh ha scritto:
>> > But as far as I can test with free servers, the FUA bits have no
>> > advantage over flush.  Also, I wasn't sure if SEND_FUA without
>> > SEND_FLUSH is valid, and if so how to handle this combination (treat it
>> > as writethrough and add FUA to all requests? warn and do nothing?).
> On the main opensource nbd client, the following applies:
> 
> What REQ_FUA does is an fdatasync() after the write. Code extract and
> comments below from Christoph Hellwig.
> 
> What REQ_FLUSH does is to do an fsync().
> 
> The way I read Christoph's comment, provided the linux block layer always
> issues a REQ_FLUSH before a REQ_FUA, there is not performance problem.
> 
> However, a REQ_FUA is going to do a f(data)?sync AFTER the write, whereas
> the preceding REQ_FLUSH is going to an fsync() BEFORE the write. It seems
> to me that either the FUA and FLUSH semantics are therefore different
> (and we need FUA), or that Christoph's comment is wrong and that you
> are guaranteed a REQ_FLUSH *after* the write with REQ_FUA.

REQ_FLUSH is indeed a flush before the write.  fdatasync is fine there too.

If you do not have REQ_FUA, as is the case with this patch, the block
layer converts it to a REQ_FLUSH *after* the write.

See block/blk-flush.c:

 * REQ_{FLUSH|FUA} requests are decomposed to sequences consisted of three
 * optional steps - PREFLUSH, DATA and POSTFLUSH - according to the request
 * properties and hardware capability.
 *
 * If the device doesn't have writeback cache, FLUSH and FUA don't make any
 * difference.  The requests are either completed immediately if there's no
 * data or executed as normal requests otherwise.
 *
 * If the device has writeback cache and supports FUA, REQ_FLUSH is
 * translated to PREFLUSH but REQ_FUA is passed down directly with DATA.
 *
 * If the device has writeback cache and doesn't support FUA, REQ_FLUSH is
 * translated to PREFLUSH and REQ_FUA to POSTFLUSH.

Paolo

  reply	other threads:[~2013-02-13 16:02 UTC|newest]

Thread overview: 20+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2013-02-12 16:06 [PATCH 0/3] NBD fixes for caching and block device flags Paolo Bonzini
2013-02-12 16:06 ` [PATCH 1/3] nbd: support FLUSH requests Paolo Bonzini
2013-02-12 17:37   ` Alex Bligh
2013-02-12 18:06     ` Paolo Bonzini
2013-02-12 21:32       ` Andrew Morton
2013-02-13  0:03         ` Alex Bligh
2013-02-13 13:00           ` Paolo Bonzini
2013-02-13 15:55             ` Alex Bligh
2013-02-13 16:02               ` Paolo Bonzini [this message]
2013-02-13 17:35                 ` Alex Bligh
2013-02-13  0:00       ` Alex Bligh
2013-02-12 22:07   ` Paul Clements
2013-02-12 16:06 ` [PATCH 2/3] nbd: fsync and kill block device on shutdown Paolo Bonzini
2013-02-12 21:41   ` Andrew Morton
2013-02-13 13:05     ` Paolo Bonzini
2013-02-12 22:15   ` Paul Clements
2013-02-12 16:06 ` [PATCH 3/3] nbd: show read-only state in sysfs Paolo Bonzini
2013-02-12 22:16   ` Paul Clements
2013-02-12 21:43 ` [PATCH 0/3] NBD fixes for caching and block device flags Andrew Morton
2013-02-13 17:14   ` [Nbd] " Wouter Verhelst

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