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Re: [Qemu-devel] [Bug 1721788] [NEW] Failed to get shared "write" lock w
From: |
Eric Blake |
Subject: |
Re: [Qemu-devel] [Bug 1721788] [NEW] Failed to get shared "write" lock with 'qemu-img info' |
Date: |
Fri, 6 Oct 2017 09:28:48 -0500 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.3.0 |
On 10/06/2017 09:09 AM, Jan Heidbrink wrote:
> Public bug reported:
>
> When running 'qemu-img info test.qcow2' while test.qcow2 is currently
> used by a Qemu process, I get the error
>
> qemu-img: Could not open 'test.qcow2': Failed to get shared "write"
> lock.
>
>
> Why does displaying information about a disk image need a write lock for the
> file?
Because there is a risk (albeit rather slight) that what you read from
the disk is inconsistent due to being an intermediate state in-between
separate non-atomic write actions by the other process that has it open
for write.
If you are willing to ignore the risk, then use:
qemu-img info -U test.qcow2
which says that you are okay reading the image while it is shared with a
concurrent writer, even if the read fails spectacularly in the unlikely
case that it sees inconsistent information.
--
Eric Blake, Principal Software Engineer
Red Hat, Inc. +1-919-301-3266
Virtualization: qemu.org | libvirt.org
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