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Re: [Qemu-devel] [PATCH] Documentation: Warn against qemu-img on active
From: |
Kevin Wolf |
Subject: |
Re: [Qemu-devel] [PATCH] Documentation: Warn against qemu-img on active image |
Date: |
Thu, 16 Aug 2012 15:35:13 +0200 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:13.0) Gecko/20120605 Thunderbird/13.0 |
Am 16.08.2012 14:56, schrieb Eric Blake:
> On 08/16/2012 04:00 AM, Peter Maydell wrote:
>> On 16 August 2012 10:00, Kevin Wolf <address@hidden> wrote:
>>> People have repeatedly expected that you can do things like snapshotting
>>> an image with qemu-img while a qemu instance is running. Maybe we need
>>> to consider locking the files while they are in use,
>
> Sounds like a nice feature bit to add to qcow2v3, where both qemu-img
> and qemu check if the locking feature is enabled for an image, as well
> as maintain a header bit that is set when the image is open read-write
> and refuse to use the image if the lock bit is set.
I thought the same. However, then you need some way to override this
mechanism when recovering from a crash etc., so it's not a trivial
addition and it would be user-visible.
>> but having a
>>> warning in the qemu-img manpage is doable for 1.2 and can't hurt anyway.
>>>
>>> Signed-off-by: Kevin Wolf <address@hidden>
>
>>
>>> +
>>> address@hidden:} Never use qemu-img to modify images in use by a running
>>> virtual
>>> +machine or any other process, this may destroy the image.
>>
>> ";" or ", because".
>
> Is this strong enough? Remember, with qcow2v3 and qed, the mere act of
> opening an image will perform refcount checks that modify the image,
> unless you explicitly request otherwise, which means even a query of the
> file metadata may result in modifying the image as part of the default
> open.
Not for read-only opens. I think qemu-img gets this right meanwhile, so
that images are opened read-only when they are only queried.
(And for qcow2v3 the check only happens with lazy refcounting enabled,
which is not the default)
> Maybe incorporate some ideas from this attempt:
>
> Never use qemu-img to modify files in use by a running virtual machine
> or any other process; this may destroy the image. Be aware that some
> image formats perform modifications even on query operations. Also, be
> aware that querying an image that is being modified by another process
> may encounter inconsistent state.
Adding the last sentence is probably a good idea anyway.
Kevin