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Re: [Qemu-devel] [PATCH 12/12 v12] introduce a new monitor command 'dump


From: Wen Congyang
Subject: Re: [Qemu-devel] [PATCH 12/12 v12] introduce a new monitor command 'dump-guest-memory' to dump guest's memory
Date: Wed, 18 Apr 2012 23:55:06 +0800
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:11.0) Gecko/20120329 Thunderbird/11.0.1

On 04/18/2012 11:35 PM, Eric Blake wrote:
On 04/18/2012 03:37 AM, Wen Congyang wrote:
   2. This command doesn't support the fd that is is associated with a pipe,
      socket, or FIFO(lseek will fail with such fd).

How hard would it be to lift that restriction ? When libvirt does save to
file, or core dump these days, we tend to pass a pipe FD to QEMU, which
is connected to libvirt's I/O helper process. The reason for this is that
it lets us turn on O_DIRECT for the dumped data, which has proved to be
quite an important feature for oVirt. So I'd rather like to keep that
ability with the new dump command.

The reason is that we will use lseek(fd, ...). If you pass a pipe FD
to qemu, lseek() will fail. I donot know the note size before we
write it to the core, so I use lseek()...

The question, then, is why we don't know the note size in advance, and
what it would take to be able to compute it in advance so that the
output file can be written in a single pass without needing lseek().


I have fixed this problem in the newest patchset. Please refer to:
http://lists.nongnu.org/archive/html/qemu-devel/2012-04/msg02493.html

Thanks
Wen Congyang



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