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

[Eric Blake]

On 08/16/2012 07:35 AM, Kevin Wolf wrote:
> 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.

Good point.  With fcntl() locking, the lock goes away on crash; but with
file modification locking, the lock can get stuck so you have to provide
overrides; and once you provide overrides, the lock is not quite as
powerful.  So maybe it's best to just leave locking up to management
apps (after all, libvirt already has a lock protocol support, currently
built on sanlock but also with a patch proposed for using fcntl()).

>> 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.

Then it might also be worth documenting which actions are read-only
queries vs. potential modifications, in the docs for each action.  (For
example, 'info' is read-only, 'convert' is read-only when creating a
copy although consistency is essential for it to be useful, 'check' is
readonly while 'check -r' is read-write,...)

-- 
Eric Blake   address@hidden    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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