From: Paolo Bonzini <pbonzini@redhat.com>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: linux-kernel@vger.kernel.org, stable@vger.kernel.org,
nbd-general@lists.sf.net,
Paul Clements <Paul.Clements@steeleye.com>
Subject: Re: [PATCH 2/3] nbd: fsync and kill block device on shutdown
Date: Wed, 13 Feb 2013 14:05:40 +0100 [thread overview]
Message-ID: <511B8FA4.1080400@redhat.com> (raw)
In-Reply-To: <20130212134155.f23f2223.akpm@linux-foundation.org>
Il 12/02/2013 22:41, Andrew Morton ha scritto:
>> > There are two problems with shutdown in the NBD driver. The first is
>> > that receiving the NBD_DISCONNECT ioctl does not sync the filesystem;
>> > this is useful because BLKFLSBUF is restricted to processes that have
>> > CAP_SYS_ADMIN, and the NBD client may not possess it (fsync of the
>> > block device does not sync the filesystem, either).
> hm, this says that the lack of a sync is "useful". I think you mean
> that the patch-which-adds-the-sync is the thing which is useful, yes?
Yes.
>> > The second is that once we clear the socket we have no guarantee that
>> > later reads will come from the same backing storage. Thus the page cache
>> > must be cleaned, lest reads that hit on the page cache will return stale
>> > data from the previously-accessible disk.
> That sounds like a problem.
>
>> > Example:
>> >
>> > # qemu-nbd -r -c/dev/nbd0 /dev/sr0
>> > # file -s /dev/nbd0
>> > /dev/stdin: # UDF filesystem data (version 1.5) etc.
>> > # qemu-nbd -d /dev/nbd0
>> > # qemu-nbd -r -c/dev/nbd0 /dev/sda
>> > # file -s /dev/nbd0
>> > /dev/stdin: # UDF filesystem data (version 1.5) etc.
>> >
>> > While /dev/sda has:
>> >
>> > # file -s /dev/sda
>> > /dev/sda: x86 boot sector; etc.
> OK, we've described the problems but there's no description here of how
> the patch addresses those problems.
>
> How does this look?
Perfect, thanks very much. I tried to similarly balance the "why" and
"how" in the new commit message for patch 1.
Paolo
next prev parent reply other threads:[~2013-02-13 13:06 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
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 [this message]
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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