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



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. 

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

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

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

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

Who/what starts the qemu-nbd process ?

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

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 ?

Regards,
Daniel
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