Re: [Qemu-devel] [PATCH] Documentation: Warn against qemu-img on active image

[Kevin Wolf <kwolf@redhat.com>]

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 <kwolf@redhat.com> 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 <kwolf@redhat.com>
> 
>>
>>> +
>>> +@b{Warning:} 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