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

From: Anthony Liguori
Subject: Re: [Qemu-devel] Qemu savevm and CPU soft lockup
Date: Wed, 23 Sep 2009 17:28:23 -0500
User-agent: Thunderbird (X11/20090825)

Jamie Lokier wrote:
Benjamin Cleyet-Marrel wrote:
   After further investigation, I figured out that when issuing a savevm
   the entire qemu process gets stuck on IO wait.
   I can't issue any other commands on the monitor the process is shown
   as D and the guest is in softlockup state.
   Looking at the way migrate (and the -d for detach I presume) is
   working I suppose the same behaviour would be expected from the savevm
   A savevm -d so that the qemu process would not be freezed while saving
   the data.
   Sorry if I am just talking non sense but my snapshot on iscsi storage
   takes about 1 minutes.
   which means that my guest are down for a minute or so which is not

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.


Anthony Liguori

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