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

From: Vladimir Sementsov-Ogievskiy
Subject: Re: [Qemu-block] [PATCH 12/22] qcow2-bitmap: add IN_USE flag
Date: Wed, 26 Oct 2016 16:02:56 +0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.4.0

26.10.2016 15:13, Vladimir Sementsov-Ogievskiy wrote:
26.10.2016 12:21, Vladimir Sementsov-Ogievskiy wrote:
26.10.2016 12:04, Vladimir Sementsov-Ogievskiy wrote:
25.10.2016 13:53, Vladimir Sementsov-Ogievskiy wrote:
24.10.2016 20:18, Max Reitz wrote:
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
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
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
    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
the image when it's opened and then overwrite them when the image is
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.

You are right, I need to handle reopening more carefully. In my way, I need to delete bitmaps when reopening R/W and in yours - set in_use bit.

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).


Ok, I'll rewrite this way, using 'in_use' bit and trying to avoid allocate/free overhead.

Trying to reuse clusters of in_use bitmaps (including contiguous allocations for bitmap tables) will complicate things a lot.. We can use extra clusters of one in_use bitmap to save another one, the same is for bitmap tables. Extra clusters of old bitmap table (in case of resize) can be used for saving other bitmap data, etc..

A compromise strategy of reusing:

1. accumulate all data clusters of in_use bitmaps, which was loaded by this qcow2 instance (we will not touch other old in_use bitmaps) to free_clusters list
2. for each persistent BdrvDirtyBitmap:
       If there is corresponding in_use bitmap in the image, and its table size == needed table size (there was no resizes), then let's reuse bitmap table.
       else, move old bitmap table clusters to free_clusters and allocate new table.
3. for each persistent BdrvDirtyBitmap:
       For bitmap data clusters, take them from free_clusters list, and if it is empty - allocate new clusters.
4. free extra clusters from free_clusters list if any.

This strategy is not optimal, but not bad I thing. Is it ok for you? (I'm not sure that this all is not premature optimization, and may be true way is to just free all old staff and reallocate it, as I do.)

With something like this it is not trivial to maintain consistency in qcow2 image in case of fail on reusing (we can get double-linked clusters, or dead links in the image on disk).. Simple solution will be to remove all in_use bitmaps from header extension, or reallocate for them bitmap tables with all zeroes. So, finally:

target: we want to reuse clusters of in_use bitmaps, which was loaded by current qcow2 driver (let's call them our in_use bitmaps.

1. save bitmap directory in memory
2. remove our in_use bitmaps from bitmap directory in disk. from this point, in case of fail we will finish with some leaking clusters and no corruptions.
3. free_clusters list = all data clusters of our in_use bitmaps
4. for each our in_use bitmap: If table size is not appropriate move bitmap table clusters to free_clusters

better is to free it, to not deceive qcow2 allocator.. Can I allocate several clusters by alloc(size), then assume that I have (size + cluster_size - 1)/cluster_size clusters and that I can free these clusters separately?

5. save bitmaps, using free_clusters list for data clusters, old existing bitmap tables and allocating new space when needed.
6. free extra clusters from free_clusters list if any

Best regards,

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