Re: [Qemu-devel] [RFC] qed: Add QEMU Enhanced Disk format

[Kevin Wolf <kwolf@redhat.com>]

Am 07.09.2010 22:41, schrieb Anthony Liguori:
> There's two types of snapshots that I think can cause confusion.  
> There's CPU/device state snapshots and then there's a block device snapshot.
> 
> qcow2 and qed both support block device snapshots.  qed only supports 
> external snapshots (via backing_file) whereas qcow2 supports external 
> and internal snapshots.  The internal snapshots are the source of an 
> incredible amount of complexity in the format.
> 
> qcow2 can also store CPU/device state snapshots and correlate them to 
> block device snapshots (within a single block device).  It only supports 
> doing non-live CPU/device state snapshots.

Which is not a property of the format, but of the implementation. I
think it shouldn't be too hard to introduce live snapshots.

> OTOH, qemu can support live snapshotting via live migration.  Today, it 
> can be used to snapshot CPU/device state to a file on the filesystem 
> with minimum downtime.
> 
> Combined with an external block snapshot and correlating data, this 
> could be used to implement a single "snapshot" command that would behave 
> like savevm but would not pause a guest's execution.

We'd need fields for referencing a VM state file from a QED image, just
like it's already done for backing files.

Kevin