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Re: [Qemu-devel] Qemu savevm and CPU soft lockup


From: Ben Accelance
Subject: Re: [Qemu-devel] Qemu savevm and CPU soft lockup
Date: Fri, 25 Sep 2009 13:31:12 +0200



Benjamin Cleyet-Marrel


Le 25 sept. 2009 à 09:40, Kevin Wolf <address@hidden> a écrit :

Am 24.09.2009 18:20, schrieb Anthony Liguori:
Kevin Wolf wrote:
Am 24.09.2009 00:28, schrieb Anthony Liguori:

Jamie Lokier wrote:

This is normal savevm behaviour, and it is exactly the reason why
migrate-to-file is useful.  I would not be surprised if savevm is
changed to use migrate-to-file internally at some point, but it does
not look like happening soon.


It's the same infrastructure.  The reason savevm isn't live is that
savevm stores it's data in a qcow2 file. Right now the way qcow2 is structured, the snapshot has to be a fixed size and allocated at once. In order to make savevm live, we need a method to stream savevm data to a qcow2 file while still allowing other IO operations to that qcow2 file.

I'm fairly sure this will require a change to the qcow2 format in order
to support this.


Hm, snapshots are nothing complicated from qcow2 perspective. Why do you
think data needs to be fixed size?

What happens if you're in the middle of writing snapshot data and
another cluster has to be allocated?  You'll need a way to store the
snapshot data discontinuously.

Well, mapping virtually continuous data to discontinuous areas in the
image file is just how qcow2 works, right?

If you look at qcow_vmstate_load/save(offset), this is basically just a call to bdrv_pread/pwrite(virtual_disk_size + offset). So for snapshots it works exactly the same way as with regular data (which usually isn't
preallocated in qcow2 images).

Kevin

Thanks for this information,
Having a live save function would be tremendous and a huge gain compare to other solutions. The migrate to file using exec is just not cutting it as I have to be in sync with image snapshot. Having said that I am still not sure I understand why savevm isn't live . or should it be ? And then it is a bug ?

Cheers
And thanks for your Time.

Ben




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