[Qemu-devel] delvm does not reduce the qcow2 file size

[Qemu-devel] delvm does not reduce the qcow2 file size

[Kuniyasu Suzaki]

Hello,

"delvm" is a command to delete a snapshot image in a qcow2 file.
However it does not reduce the qcow2 file size.

Is there any method to reduce the qcow2 file size when I delete a snapshot image?
should I "convert" the qcow2 file?

------
suzaki

Re: [Qemu-devel] delvm does not reduce the qcow2 file size

[Paolo Bonzini]

Il 15/10/2012 11:33, Kuniyasu Suzaki ha scritto:
> Hello,
> 
> "delvm" is a command to delete a snapshot image in a qcow2 file.
> However it does not reduce the qcow2 file size.

Note that the file will not grow when new allocations are performed in
the future.

QEMU should also convert the unused clusters to "holes" in the file
system, but it doesn't do that yet.

> Is there any method to reduce the qcow2 file size when I delete a snapshot image?
> should I "convert" the qcow2 file?

I'm not sure what convert does if you have internal snapshots.

Paolo

Re: [Qemu-devel] delvm does not reduce the qcow2 file size

[Kevin Wolf]

Am 15.10.2012 15:25, schrieb Paolo Bonzini:
> Il 15/10/2012 11:33, Kuniyasu Suzaki ha scritto:
>> Hello,
>>
>> "delvm" is a command to delete a snapshot image in a qcow2 file.
>> However it does not reduce the qcow2 file size.
> 
> Note that the file will not grow when new allocations are performed in
> the future.
> 
> QEMU should also convert the unused clusters to "holes" in the file
> system, but it doesn't do that yet.
> 
>> Is there any method to reduce the qcow2 file size when I delete a snapshot image?
>> should I "convert" the qcow2 file?
> 
> I'm not sure what convert does if you have internal snapshots.

It loses the snapshots.

Kevin

Re: [Qemu-devel] delvm does not reduce the qcow2 file size

[Kuniyasu Suzaki]

Hello,

From: Paolo Bonzini <pbonzini@redhat.com>
Subject: Re: delvm does not reduce the qcow2 file size
Date: Mon, 15 Oct 2012 15:25:16 +0200

> Il 15/10/2012 11:33, Kuniyasu Suzaki ha scritto:
> > Hello,
> > 
> > "delvm" is a command to delete a snapshot image in a qcow2 file.
> > However it does not reduce the qcow2 file size.
> 
> Note that the file will not grow when new allocations are performed in
> the future.

You mean that the qcow2 file size will not increase when "savevm"
commnad is issued after "delvm".
Namely, the qcow2 file size is almost same when I take a snapshot
after delete previous snapshot image.

> QEMU should also convert the unused clusters to "holes" in the file
> system, but it doesn't do that yet.

I hope a qcow2 file is reduced automatically when a snapshot is deleted.

> > Is there any method to reduce the qcow2 file size when I delete a snapshot image?
> > should I "convert" the qcow2 file?
> 
> I'm not sure what convert does if you have internal snapshots.
> 
> Paolo

------
suzaki

Re: [Qemu-devel] delvm does not reduce the qcow2 file size

[Paolo Bonzini]

Il 16/10/2012 08:23, Kuniyasu Suzaki ha scritto:
>>> "delvm" is a command to delete a snapshot image in a qcow2 file.
>>> However it does not reduce the qcow2 file size.
>>
>> Note that the file will not grow when new allocations are performed in
>> the future.
> 
> You mean that the qcow2 file size will not increase when "savevm"
> commnad is issued after "delvm".
> Namely, the qcow2 file size is almost same when I take a snapshot
> after delete previous snapshot image.

Exactly.  The first writes to the qcow2 file will also reuse the space
left free by "delvm", if they have to allocate new clusters.

>> QEMU should also convert the unused clusters to "holes" in the file
>> system, but it doesn't do that yet.
> 
> I hope a qcow2 file is reduced automatically when a snapshot is deleted.

Yes, that's part of the plan.  The file would not reduce its size, but
the space would still be given back to the filesystem.

Paolo

Re: [Qemu-devel] delvm does not reduce the qcow2 file size

[Kuniyasu Suzaki]

Hello,

From: Paolo Bonzini <pbonzini@redhat.com>
Subject: Re: delvm does not reduce the qcow2 file size
Date: Tue, 16 Oct 2012 08:54:46 +0200

> >> QEMU should also convert the unused clusters to "holes" in the file
> >> system, but it doesn't do that yet.
> > 
> > I hope a qcow2 file is reduced automatically when a snapshot is deleted.
> 
> Yes, that's part of the plan.  The file would not reduce its size, but
> the space would still be given back to the filesystem.

Do you plan to use a space file system?
Is it difficult to reduce a qcow2 file when a snapshot is deleted?

------
suzaki

Re: [Qemu-devel] delvm does not reduce the qcow2 file size

[Paolo Bonzini]

Il 16/10/2012 13:06, Kuniyasu Suzaki ha scritto:
>>>> QEMU should also convert the unused clusters to "holes" in the file
>>>> > >> system, but it doesn't do that yet.
>>> > > 
>>> > > I hope a qcow2 file is reduced automatically when a snapshot is deleted.
>> > 
>> > Yes, that's part of the plan.  The file would not reduce its size, but
>> > the space would still be given back to the filesystem.
> Do you plan to use a space file system?

Any file system that support holes, including ext2/3/4 or btrfs or XFS,
would actually reclaim the space.

> Is it difficult to reduce a qcow2 file when a snapshot is deleted?

There is no guarantee that no data was allocated after the snapshot.  If
it was, it is not possible to reduce the qcow2 file's size without
implementing some kind of "defragmentation".

Paolo