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Re: [Qemu-devel] qemu-img commit -- is there a limit on file sizes?

From: walt
Subject: Re: [Qemu-devel] qemu-img commit -- is there a limit on file sizes?
Date: Mon, 1 Dec 2008 16:47:55 -0800
User-agent: Alpine 2.00 (LNX 1169 2008-08-27)

On Mon, 1 Dec 2008, Avi Kivity wrote:

> Anthony Liguori wrote:
> >
> > We've started getting some reports of corruption on "commit" in KVM.  There
> > is a long standing disk corruption issue too that is very difficult to
> > reproduce.  The thinking is that there is a bug somewhere in the qcow2 code.
> >
> > Is anyone actively looking into this?
> >
> I am, though my actively is a lot less than could be desired.  Additional eyes
> would be welcome.

FWIW, I must apologize for giving you incorrect data.  I'm seeing problems
now that have nothing to do with the size of the commit, and I'm beginning
to suspect that the commit step has nothing to do with the problem.  I'll
summarize my evidence because it seems potentially very important:

I installed WinXP on qcow2, which went perfectly.  I rebooted multiple times
with no problems and changed settings for my desktop and taskbar, rebooted
again with no problems.

Now, I make a new, fresh [what's the opposite of a backing file?] like this:
$qemu-img create -f qcow2 -b kvmXP kvmXP.delta

All I do is boot XP again like this:
$qemu-system-x86_64 -m 1000 kvmXP.delta

My shock is that one of the taskbar settings I changed has disappeared!
I can boot the original kvmXP qcow2 image and verify that my changes are
still there in the original, but not when I boot kvmXP.delta.

In case someone wants to try to reproduce it, the specific change I made
to the task bar is to display the animated network activity icon in the
system tray.  That icon never appears when I boot from kvmXP.delta.

In other words, from the instant I boot kvmXP.delta, the image of XP that
gets loaded into memory is not an accurate reflection of what's in the
backing file.  If that's true, then it's not surprising that the commit
step causes trouble.

Does my reasoning seem reasonable? :o)

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