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Re: [Qemu-devel] [RFC] qmp interface for save vmstate to image
From: |
Stefan Hajnoczi |
Subject: |
Re: [Qemu-devel] [RFC] qmp interface for save vmstate to image |
Date: |
Mon, 18 Mar 2013 11:09:10 +0100 |
User-agent: |
Mutt/1.5.21 (2010-09-15) |
On Mon, Mar 18, 2013 at 02:40:50PM +0800, Wenchao Xia wrote:
> δΊ 2013-3-15 22:51, Stefan Hajnoczi ει:
> > On Fri, Mar 15, 2013 at 03:24:38PM +0800, Wenchao Xia wrote:
> >> I'd like to add a new way to save vmstate, which will based on the
> >> migration thread, but will write contents to block images, instead
> >> of fd as stream. Following is the method to add API:
> >
> > Hi Wenchao,
> > What use cases are there besides saving vmstate to a raw image?
> >
> > I'm curious if you're proposing this since there is no "file:" URI or
> > because you really want to do things like saving vmstate into a qcow2
> > file or over NBD.
> >
> > Stefan
> >
> Hi, Stefan
> Most used cases would be "raw" and "qcow2", which is flex and can be
> chosen by user. In this way, existing block layer feature in qemu can
> be used, such as tagging zeros. I haven't check the buffer/cache status
> in qemu block layer, but if there is, it can also benefit.
Okay, thanks for explaining.
You can use caching with the BDRV_O_CACHE_WB option. Then you need to
call bdrv_co_flush() to ensure data reaches the disk. The advantage of
caching is that I/O patterns with many small unaligned writes may be
much faster when going through the host's page cache - and reads can
also be faster.
You can bypass the host page cache with BDRV_O_CACHE_WB | BDRV_NO_CACHE.
Here bdrv_co_flush() calls are still necessary to ensure data reaches
the disk.
Stefan
Re: [Qemu-devel] [RFC] qmp interface for save vmstate to image, Pavel Hrdina, 2013/03/18
- Re: [Qemu-devel] [RFC] qmp interface for save vmstate to image, Wenchao Xia, 2013/03/21
- Re: [Qemu-devel] [RFC] qmp interface for save vmstate to image, Pavel Hrdina, 2013/03/21
- Re: [Qemu-devel] [RFC] qmp interface for save vmstate to image, Stefan Hajnoczi, 2013/03/21
- Re: [Qemu-devel] [RFC] qmp interface for save vmstate to image, Paolo Bonzini, 2013/03/21
- Re: [Qemu-devel] [RFC] qmp interface for save vmstate to image, Pavel Hrdina, 2013/03/21
- Re: [Qemu-devel] [RFC] qmp interface for save vmstate to image, Stefan Hajnoczi, 2013/03/21
- Re: [Qemu-devel] [RFC] qmp interface for save vmstate to image, Eric Blake, 2013/03/21
- Re: [Qemu-devel] [RFC] qmp interface for save vmstate to image, Wenchao Xia, 2013/03/23
- Re: [Qemu-devel] [RFC] qmp interface for save vmstate to image, Wenchao Xia, 2013/03/26