Re: [Qemu-devel] [PATCH 12/22] qcow2-bitmap: add IN_USE flag

[Max Reitz]

On 24.10.2016 19:08, Max Reitz wrote:
> On 24.10.2016 13:35, Vladimir Sementsov-Ogievskiy wrote:
>> 24.10.2016 13:32, Vladimir Sementsov-Ogievskiy пишет:
>>> 21.10.2016 22:58, Max Reitz пишет:
>>>> On 21.10.2016 17:34, Vladimir Sementsov-Ogievskiy wrote:
>>>>> 07.10.2016 22:44, Max Reitz пишет:
>>>>>> On 30.09.2016 12:53, Vladimir Sementsov-Ogievskiy wrote:
>>>>>>> This flag means that the bitmap is now in use by the software or
>>>>>>> was not
>>>>>>> successfully saved. In any way, with this flag set the bitmap data
>>>>>>> must
>>>>>>> be considered inconsistent and should not be loaded.
>>>>>>>
>>>>>>> With current implementation this flag is never set, as we just remove
>>>>>>> bitmaps from the image after loading. But it defined in qcow2 spec
>>>>>>> and
>>>>>>> must be handled. Also, it can be used in future, if async schemes of
>>>>>>> bitmap loading/saving are implemented.
>>>>>>>
>>>>>>> We also remove in-use bitmaps from the image on open.
>>>>>>>
>>>>>>> Signed-off-by: Vladimir Sementsov-Ogievskiy
>>>>>>> <address@hidden>
>>>>>>> ---
>>>>>>>    block/qcow2-bitmap.c | 17 ++++++++++++++++-
>>>>>>>    1 file changed, 16 insertions(+), 1 deletion(-)
>>>>>> Don't you want to make use of this flag? It would appear useful to
>>>>>> me if
>>>>>> you just marked autoload bitmaps as in_use instead of deleting them
>>>>>> from
>>>>>> the image when it's opened and then overwrite them when the image is
>>>>>> closed.
>>>>> And what is the use of it?
>>>> You don't need to free any bitmaps when opening the file, and you don't
>>>> need to allocate any new bitmap space when closing it.
>>>
>>> As bitmaps are sparce in file, I need to allocate new space when
>>> closing. Or free it...
>>
>>
>> Is it a real problem to reallocate space in qcow2? If so, to minimaze
>> allocations, I will have to make a list of clusters of in_use bitmaps on
>> close, and then save current bitmaps to these clusters (and allocated
>> some additional clusters, or free extra clusters).
> 
> It's not a real problem, but it does take time, and I maintain that it's
> time it doesn't need to take because you can just use the in_use flag.
> 
> I wouldn't worry about reusing clusters of other bitmaps. Of course we
> can do that later on in some optimization but not now.
> 
> I just mean the basic case of some auto-loaded bitmap that is not being
> deleted while the VM is running and is just saved back to the image file
> once it is closed. I don't expect that users will always consume all of
> the auto-loaded bitmaps while the VM is active...
> 
>> Also, if I don't free them on open, I'll have to free them on remove of
>> bdrv dirty bitmap..
> 
> Yes, so? That takes time, but that is something the user will probably
> expect. I wouldn't expect opening of the file to take that time.
> 
> Overall, it doesn't matter time-wise whether you free the bitmap data
> when opening the image file or when the dirty bitmap is actually deleted
> by the user or when the image file is closed. Just setting the single
> in_use flag for all of the auto-loaded bitmaps will basically not take
> any time.
> 
> On the other hand, as soon as just a single auto-loaded bitmap survives
> a VM (or qemu-img) invocation, you will very, very likely safe at least
> some time because writing the bitmap to the disk can reuse at least some
> of the existing clusters.

By the way, dealing with removal of bitmaps when they are deleted by the
user is also positive when it comes to migration or read-only disks that
are later reopened R/W: Currently, as far as I can see, you just keep
the auto-load bitmaps in the image if the image is part of an incoming
migration or opened read-only, but then you continue under the
assumption that they are removed from the image. That's not very good.

If you delete the bitmaps only when the user asks you to, then you can
just return an error that the bitmap cannot be removed at this time
because the image file is read-only or used in migration (and I don't
think the latter can even happen when it's the user who has requested
removal of the bitmap).

Max

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