From: John Snow <jsnow@redhat.com>
To: Max Reitz <mreitz@redhat.com>, qemu-block@nongnu.org
Cc: Kevin Wolf <kwolf@redhat.com>,
Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>,
qemu-devel@nongnu.org
Subject: Re: [Qemu-devel] [Qemu-block] [PATCH 0/3] block: Make various formats' block_status recurse again
Date: Mon, 12 Aug 2019 14:39:10 -0400 [thread overview]
Message-ID: <125926bf-4c5a-939a-2cc7-01e11a1a2511@redhat.com> (raw)
In-Reply-To: <20190725155512.9827-1-mreitz@redhat.com>
On 7/25/19 11:55 AM, Max Reitz wrote:
> Hi,
>
> 69f47505ee66afaa513305de0c1895a224e52c45 changed block_status so that it
> would only go down to the protocol layer if the format layer returned
> BDRV_BLOCK_RECURSE, thus indicating that it has no sufficient
> information whether a given range in the image is zero or not.
> Generally, this is because the image is preallocated and thus all ranges
> appear as zeroes.
>
> However, it only implemented this preallocation detection for qcow2.
> There are more formats that support preallocation, though: vdi, vhdx,
> vmdk, vpc. (Funny how they all start with “v”.)
>
> For vdi, vmdk, and vpc, the fix is rather simple, because they really
> have different subformats depending on whether an image is preallocated
> or not. This makes the check very simple.
>
> vhdx is more like qcow2, where after the image has been created, it
> isn’t clear whether it’s been preallocated or everything is allocated
> because everything was already written to. 69f47505ee added a heuristic
> to qcow2 to get around this, but I think that’s too much for vhdx. I
> just left it unfixed, because I don’t care that much, honestly (and I
> don’t think anyone else does).
>
What's the practical outcome of that, and is the limitation documented
somewhere?
(I'm fine with not fixing it, I just want it documented somehow.)
>
> Max Reitz (3):
> vdi: Make block_status recurse for fixed images
> vmdk: Make block_status recurse for flat extents
> vpc: Do not return RAW from block_status
>
> block/vdi.c | 3 ++-
> block/vmdk.c | 3 +++
> block/vpc.c | 2 +-
> 3 files changed, 6 insertions(+), 2 deletions(-)
>
next prev parent reply other threads:[~2019-08-12 18:39 UTC|newest]
Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-07-25 15:55 [Qemu-devel] [PATCH 0/3] block: Make various formats' block_status recurse again Max Reitz
2019-07-25 15:55 ` [Qemu-devel] [PATCH 1/3] vdi: Make block_status recurse for fixed images Max Reitz
2019-08-12 14:47 ` Vladimir Sementsov-Ogievskiy
2019-07-25 15:55 ` [Qemu-devel] [PATCH 2/3] vmdk: Make block_status recurse for flat extents Max Reitz
2019-08-12 14:59 ` Vladimir Sementsov-Ogievskiy
2019-07-25 15:55 ` [Qemu-devel] [PATCH 3/3] vpc: Do not return RAW from block_status Max Reitz
2019-08-12 15:33 ` Vladimir Sementsov-Ogievskiy
2019-08-12 15:56 ` Max Reitz
2019-08-12 16:50 ` Vladimir Sementsov-Ogievskiy
2019-08-12 19:07 ` Max Reitz
2019-08-13 10:38 ` Kevin Wolf
2019-08-13 14:49 ` Max Reitz
2019-08-12 18:39 ` John Snow [this message]
2019-08-12 19:11 ` [Qemu-devel] [Qemu-block] [PATCH 0/3] block: Make various formats' block_status recurse again Max Reitz
2019-08-12 21:45 ` John Snow
2019-08-13 14:48 ` Max Reitz
2019-08-13 22:35 ` John Snow
2019-08-15 15:49 ` [Qemu-devel] " Max Reitz
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