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From: Anton Nefedov <anton.nefedov@virtuozzo.com>
To: Eric Blake <eblake@redhat.com>, qemu-devel@nongnu.org
Cc: kwolf@redhat.com, "Denis V . Lunev" <den@openvz.org>,
	den@virtuozzo.com, mreitz@redhat.com
Subject: Re: [Qemu-devel] [PATCH v1 03/13] qcow2: do not COW the empty areas
Date: Tue, 23 May 2017 11:31:51 +0300	[thread overview]
Message-ID: <53413715-77ed-0b86-c893-df8575c302ce@virtuozzo.com> (raw)
In-Reply-To: <fb751647-3473-76e2-bc49-bf2c9734a027@redhat.com>



On 05/22/2017 10:24 PM, Eric Blake wrote:
> On 05/19/2017 04:34 AM, Anton Nefedov wrote:
>> If COW area of the newly allocated cluster is zeroes, there is no reason
>> to write zero sectors in perform_cow() again now as whole clusters are
>> zeroed out in single chunks by handle_alloc_space().
>
> But that's only true if you can guarantee that handle_alloc_space()
> succeeded at ensuring the cluster reads as zeroes.  If you silently
> ignore errors (which is what patch 1/13 does), you risk assuming that
> the cluster reads as zeroes when in reality it does not, and then you
> have corrupted data.
>

Sure; COW is only skipped if pwrite_zeroes() from patch 1/13
succeeds

> The idea of avoiding a COW of areas that read as zero at the source when
> the destination also already reads as zeroes makes sense, but I'm not
> convinced that this patch is safe as written.
>

/Anton

  reply	other threads:[~2017-05-23  8:32 UTC|newest]

Thread overview: 36+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-05-19  9:34 [Qemu-devel] [PATCH v1 00/13] qcow2: space preallocation and COW improvements Anton Nefedov
2017-05-19  9:34 ` [Qemu-devel] [PATCH v1 01/13] qcow2: alloc space for COW in one chunk Anton Nefedov
2017-05-22 19:00   ` Eric Blake
2017-05-23  8:28     ` Anton Nefedov
2017-05-23  9:13     ` Denis V. Lunev
2017-05-26  8:11   ` Kevin Wolf
2017-05-26  8:57     ` Denis V. Lunev
2017-05-26 10:09       ` Anton Nefedov
2017-05-26 11:16       ` Kevin Wolf
2017-05-26 10:57     ` Denis V. Lunev
2017-05-26 11:32       ` Kevin Wolf
2017-05-19  9:34 ` [Qemu-devel] [PATCH v1 02/13] qcow2: is_zero_sectors(): return true if area is outside of backing file Anton Nefedov
2017-05-22 19:12   ` Eric Blake
2017-05-22 19:14     ` Eric Blake
2017-05-23  8:35       ` Anton Nefedov
2017-05-19  9:34 ` [Qemu-devel] [PATCH v1 03/13] qcow2: do not COW the empty areas Anton Nefedov
2017-05-22 19:24   ` Eric Blake
2017-05-23  8:31     ` Anton Nefedov [this message]
2017-05-23  9:15     ` Denis V. Lunev
2017-05-19  9:34 ` [Qemu-devel] [PATCH v1 04/13] qcow2: preallocation at image expand Anton Nefedov
2017-05-22 19:29   ` Eric Blake
2017-05-24 16:57     ` Anton Nefedov
2017-05-19  9:34 ` [Qemu-devel] [PATCH v1 05/13] qcow2: set inactive flag Anton Nefedov
2017-05-26  8:11   ` Kevin Wolf
2017-05-31 16:56     ` Anton Nefedov
2017-05-19  9:34 ` [Qemu-devel] [PATCH v1 06/13] qcow2: truncate preallocated space Anton Nefedov
2017-05-19  9:34 ` [Qemu-devel] [PATCH v1 07/13] qcow2: check space leak at the end of the image Anton Nefedov
2017-05-19  9:34 ` [Qemu-devel] [PATCH v1 08/13] qcow2: handle_prealloc(): find out if area zeroed by earlier preallocation Anton Nefedov
2017-05-19  9:34 ` [Qemu-devel] [PATCH v1 09/13] qcow2: fix misleading comment about L2 linking Anton Nefedov
2017-05-19  9:34 ` [Qemu-devel] [PATCH v1 10/13] qcow2-cluster: slightly refactor handle_dependencies() Anton Nefedov
2017-05-19  9:34 ` [Qemu-devel] [PATCH v1 11/13] qcow2-cluster: make handle_dependencies() logic easier to follow Anton Nefedov
2017-05-22 19:37   ` Eric Blake
2017-05-19  9:34 ` [Qemu-devel] [PATCH v1 12/13] qcow2: allow concurrent unaligned writes to the same clusters Anton Nefedov
2017-05-19  9:34 ` [Qemu-devel] [PATCH v1 13/13] iotest 046: test simultaneous cluster write error case Anton Nefedov
2017-05-23 14:35 ` [Qemu-devel] [PATCH v1 00/13] qcow2: space preallocation and COW improvements Eric Blake
2017-05-23 14:51   ` Denis V. Lunev

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