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Re: [PATCH v2] qcow2: Fix preallocation on images with unaligned sizes
From: |
Alberto Garcia |
Subject: |
Re: [PATCH v2] qcow2: Fix preallocation on images with unaligned sizes |
Date: |
Tue, 23 Jun 2020 16:14:29 +0200 |
User-agent: |
Notmuch/0.18.2 (http://notmuchmail.org) Emacs/24.4.1 (i586-pc-linux-gnu) |
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On Wed 17 Jun 2020 04:00:36 PM CEST, Alberto Garcia wrote:
> When resizing an image with qcow2_co_truncate() using the falloc or
> full preallocation modes the code assumes that both the old and new
> sizes are cluster-aligned.
>
> There are two problems with this:
>
> 1) The calculation of how many clusters are involved does not always
> get the right result.
>
> Example: creating a 60KB image and resizing it (with
> preallocation=full) to 80KB won't allocate the second cluster.
>
> 2) No copy-on-write is performed, so in the previous example if
> there is a backing file then the first 60KB of the first cluster
> won't be filled with data from the backing file.
>
> This patch fixes both issues.
>
> Signed-off-by: Alberto Garcia <berto@igalia.com>