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Date: Thu, 02 Apr 2015 13:58:51 +0300
From: Michael Tokarev <mjt@tls.msk.ru>
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Subject: Re: [Qemu-devel] block-commit & dropping privs
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To: Kevin Wolf <kwolf@redhat.com>
Cc: qemu-devel <qemu-devel@nongnu.org>, qemu-block@nongnu.org

01.04.2015 15:34, Kevin Wolf wrote:
[]
> Overriding the backing file should work like this:
> 
>     -drive file=...,backing.file.filename=/dev/fdset/2

Oh-ok, this works.  Sort of.

Because after performing commit (is there a difference between
commit hmp command and block-commit qmp command?  I used the
former so far), the new backing file name is recorded to the
qcow2 file header, and now, qemu-img can't operate on this
file anymore, telling me it can't access backing file due
to bad file descriptor.

So in order to finally commit the file I used qemu-system
again, running it like that specifying backing file, but
with -S option, and issued `commit' command again.

I noticed that I can't specify backing file for
qemu-img commit.

But overall, I think qemu-system should not modify backing
file name in this case.

When performing commit, does qemu mark the areas in the
overlay file as free after writing contents to the backing
file, or will these areas be written again by a subsequent
commit?  Somehow it smells like each next commit writes
more and more data and completes in more and more time.

Thanks,

/mjt