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Re: [Qemu-devel] [PATCH 00/13] QED image streaming
From: |
Stefan Hajnoczi |
Subject: |
Re: [Qemu-devel] [PATCH 00/13] QED image streaming |
Date: |
Wed, 15 Jun 2011 13:18:14 +0100 |
On Wed, Jun 15, 2011 at 11:46 AM, Philipp Hahn <address@hidden> wrote:
> On Tuesday 14 June 2011 20:18:18 Stefan Hajnoczi wrote:
>> This patch series adds image streaming support for QED image files. Other
>> image formats can also be supported in the future.
>>
>> Image streaming populates the file in the background while the guest is
>> running. This makes it possible to start the guest before its image file
>> has been fully provisioned.
> ...
>> When image streaming is enabled, the unallocated regions of the image file
>> are populated with the data from the backing file. This occurs in the
>> background and the guest can perform regular I/O in the meantime. Once the
>> entire backing file has been streamed, the image no longer requires a
>> backing file and will drop its reference.
>
> Does this also work for intermediate images, that is
> - master image on a NFS share,
> - local copy on a local drive using CoR-streaming from master,
> - local instance based on local copy using CoW.
>
> We normally have mostly independent servers without a shared storage, expect a
> global NFS share for the master images. Currently we have to copy the master
> image to the local server first, before we than can create lots of instances
> from then, which have few changed relative to the master, so we don't want
> the copying to happen there.
No, unfortunately adding another level of backing files prevents use
of streaming today. The reason is because backing files are opened
read-only. This ensures that multiple VMs running concurrently will
not change the backing file. Image formats are typically not safe for
concurrent updates from multiple applications just like regular file
systems cannot be accessed from multiple hosts simultaneously. You'd
need a cluster file system for that, and similarly we need multiple
writers support for image formats or some other solution.
We'd need a solution that makes it possible to update the local copy
of the master image while multiple VMs are running. For example
qemu-nbd serving the local copy of the master image and each VM image
using the backing file over NBD. That way all accesses to the local
copy of the master image go through qemu-nbd and streaming can be
performed safely. I haven't tested this approach, it might require
some improvements to qemu-nbd and I expect the performance would need
work too.
Thanks for raising this use case, I think it's an interesting one
because you are saving disk space by keeping a local master copy.
Stefan
- [Qemu-devel] [PATCH 13/13] trace: trace bdrv_aio_readv/writev error paths, (continued)
- [Qemu-devel] [PATCH 13/13] trace: trace bdrv_aio_readv/writev error paths, Stefan Hajnoczi, 2011/06/14
- [Qemu-devel] [PATCH 05/13] qed: make qed_aio_write_alloc() reusable, Stefan Hajnoczi, 2011/06/14
- [Qemu-devel] [PATCH 12/13] qed: intelligent streaming implementation, Stefan Hajnoczi, 2011/06/14
- [Qemu-devel] [PATCH 08/13] qerror: add qerror_from_args() to create qerror objects, Stefan Hajnoczi, 2011/06/14
- [Qemu-devel] [PATCH 10/13] qmp: add QMP support for stream commands, Stefan Hajnoczi, 2011/06/14
- [Qemu-devel] [PATCH 11/13] block: add -drive stream=on|off, Stefan Hajnoczi, 2011/06/14
- [Qemu-devel] [PATCH 03/13] qed: replace is_write with flags field, Stefan Hajnoczi, 2011/06/14
- [Qemu-devel] [PATCH 01/13] qemu-config: }, { -> }, { to please checkpatch.pl, Stefan Hajnoczi, 2011/06/14
- [Qemu-devel] [PATCH 02/13] block: add -drive copy-on-read=on|off, Stefan Hajnoczi, 2011/06/14
- Re: [Qemu-devel] [PATCH 00/13] QED image streaming, Philipp Hahn, 2011/06/15
- Re: [Qemu-devel] [PATCH 00/13] QED image streaming,
Stefan Hajnoczi <=
- [Qemu-devel] Image streaming and live block copy (was: [PATCH 00/13] QED image streaming), Kevin Wolf, 2011/06/16
- Re: [Qemu-devel] Image streaming and live block copy, Anthony Liguori, 2011/06/16
- Re: [Qemu-devel] Image streaming and live block copy, Avi Kivity, 2011/06/16
- Re: [Qemu-devel] Image streaming and live block copy, Kevin Wolf, 2011/06/16
- Re: [Qemu-devel] Image streaming and live block copy, Avi Kivity, 2011/06/16
- Re: [Qemu-devel] Image streaming and live block copy, Marcelo Tosatti, 2011/06/16
- Re: [Qemu-devel] Image streaming and live block copy, Stefan Hajnoczi, 2011/06/16
- Re: [Qemu-devel] Image streaming and live block copy, Marcelo Tosatti, 2011/06/17
- Re: [Qemu-devel] Image streaming and live block copy, Marcelo Tosatti, 2011/06/17