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Re: [Qemu-devel] [PATCH v4 00/27] block: Lock images when opening

From: Richard W.M. Jones
Subject: Re: [Qemu-devel] [PATCH v4 00/27] block: Lock images when opening
Date: Tue, 10 May 2016 11:16:12 +0100
User-agent: Mutt/1.5.20 (2009-12-10)

On Tue, May 10, 2016 at 12:07:06PM +0200, Kevin Wolf wrote:
> I'm surprised how low the standards seem to be when we're talking about
> data integrity. If occasionally losing data is okay, the qemu block
> layer could be quite a bit simpler.

I welcome this patch because it fixes a real data integrity issue
which we've seen in the field: people using guestfish (in write mode)
on live VMs.

We try our very best to prevent this happening -- for example if you
use guestfish via libvirt, it will check if the VM is live and refuse
access.  Though this is not and cannot be bulletproof (since someone
can start a VM up after we have opened it).  We also have prominent
warnings in the manual and in the FAQ about this.

However _reading_ disks doesn't corrupt live VMs.  The worst that
happens is guestfish will error out or you'll see some inconsistent
stats from virt-df.

None of this has anything to do with data integrity in the qemu block
layer, and no one is arguing that it should be weakened.


Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
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