[libvirt] [RFC] Proposed API to support block device streaming

Adam Litke agl at us.ibm.com
Wed Nov 10 14:45:20 UTC 2010

On Wed, 2010-11-10 at 11:33 +0000, Daniel P. Berrange wrote:
> On Tue, Nov 09, 2010 at 03:17:23PM -0600, Adam Litke wrote:
> > I've been working with Anthony Liguori and Stefan Hajnoczi to enable data
> > streaming to copy-on-read disk images in qemu.  This work is working its way
> > through peer review and I expect it to be upstream soon as part of the support
> > for the new QED disk image format.
> > 
> > I would like to enable these commands in libvirt in order to support at least
> > two compelling use cases:
> > 
> > 1) Rapid deployment of domains:
> > Creating a new domain from a central repository of images can be time consuming
> > since a local copy of the image must be made before the domain can be started.
> > With copy-on-read and streaming, up-front copy time is eliminated and the
> > domain can be started immediately.  Streaming can run while the domain runs
> > to fully populate the disk image.
> > 
> > 2) Post-copy live block migration:
> > A qemu-nbd server is started on the source host and serves the domain's block
> > device to the destination host.  A QED image is created on the destination host
> > with backing to the nbd server.  The domain is migrated as normal.  When
> > migration completes, a stream command is executed to fully populate the
> > destination QED image.  After streaming completes, the qemu-nbd server can
> > be shut down and the domain (including local storage) is fully independent of
> > the source host.
> > 
> > Qemu will support two streaming modes: full device and single sector.  Full
> > device streaming is the easiest to use because one command will cause the whole
> > device to be streamed as fast as possible.  Single sector mode can be used if
> > one wants to throttle streaming to reduce I/O pressure.  In this mode, the user
> > issues individual commands to stream single sectors.
> > 
> > To enable this support in libvirt, I propose the following API...
> > 
> > virDomainStreamDisk() initiates either a full device stream or a single sector
> > stream (depending on virDomainStreamDiskFlags).  For a full device stream, it
> > returns either 0 or -1.  For a single sector stream, it returns an offset that
> > can be used to continue streaming with a subsequent call to virDomainStreamDisk().
> > 
> > virDomainStreamDiskInfo() returns the status of a currently-running full device
> > stream (the device name, current streaming position, and total size).
> > 
> > Comments on this design would be greatly appreciated.  Thanks!
> I'm finding it hard to say whether these APIs are suitable or not
> because I can't see what this actually maps to in terms of
> implementation. 

Please see the qemu driver piece that I will post as a reply to this
email.  Since I am not looking for any particular code review at this
point I decided not to post the whole series.  But I would be happy to
do so.

> Do these calls need to be run before the QEMU process is started,
> or after QEMU is already running ?

Streaming requires a running domain and runs concurrently.

> Does the path in the arg actually need to exist on disk before 
> streaming begins, or do these APIs create the image too ?

The path actually refers to the alias of the currently attached disk
(which must be a copy-on-read disk).  For example: 'drive-virtio-disk0'.
When started, the stream command will populate the local image file with
blocks from the backing file until the local file is complete and the
backing_file link can be broken.

> If we're streaming the whole disk, is there a way to cancel/abort 
> it early ? 

I was thinking of adding another mode flag for this:

> What happens if qemu-nbd dies before streaming is complete ? 

Bad things.  Same as if you deleted a qcow2 backing file.

> Who/what starts the qemu-nbd process ?

This API doesn't yet implement any kind of migration workflow (but that
is next on my plate).  As currently designed, an external entity would
prepare nbd server on the source machine and create the target block
device on the destination host (linked to the nbd server).  Once these
two things are set up, the normal libvirt migration workflow can be
used.  On the destination machine, the stream command would then be used
to expediently remove the domain's dependency on the nbd-served base

> If you have a guest on host A and want to migrate to host B, we presumably
> need to start qemu-nbd on host A, while the guest is still running on
> host A. eg we end up with 2 processes having the same disk image open on
> host A for a while.


> How we'd wire qemu-nbd up into the security driver framework is of 
> particular concern here, because I'd think we'd want qemu-nbd to run
> wit hthe same privileges as the qemu, so that its isolated from all
> other QEMU processes on the host and can only access the one set of
> disks  for that VM

This would be for the block-migration workflow...  I can't see any
particular problem with running qemu-nbd as a regular user.  That's how
I do it when testing.

> Is there any restriction on what can be done while streaming is taking
> place ? eg if I'm doing a whole disk stream, can I migrate the QEMU
> guest to another host before streaming completes ?

The domain can be rebooted, paused, and shutdown since streaming runs
below the purview of the guest machine. 

Chained migrations would be a fun test to try but if set up properly it
should work.  The trickiest part would be knowing when it's safe to
retire the nbd servers.   


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