[Top][All Lists]
[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
Re: [Qemu-devel] Re: qcow2 corruption observed, fixed by reverting old c
From: |
Jamie Lokier |
Subject: |
Re: [Qemu-devel] Re: qcow2 corruption observed, fixed by reverting old change |
Date: |
Sun, 15 Feb 2009 02:37:18 +0000 |
User-agent: |
Mutt/1.5.13 (2006-08-11) |
Marc Bevand wrote:
> On Fri, Feb 13, 2009 at 8:23 AM, Jamie Lokier <address@hidden> wrote:
> >
> > Marc.. this is quite a serious bug you've reported. Is there a
> > reason you didn't report it earlier?
>
> Because I only started hitting that bug a couple weeks ago after
> having upgraded to a buggy kvm version.
>
> > Is there a way to restructure the code and/or how it works so it's
> > more clearly correct?
>
> I am seriously concerned about the general design of qcow2. The code
> base is more complex than it needs to be, the format itself is
> susceptible to race conditions causing cluster leaks when updating
> some internal datastructures, it gets easily fragmented, etc.
When I read it, I thought the code was remarkably compact for what it
does, although I agree that the leaks, fragmentation and inconsistency
on crashes are serious. From elsewhere it sounds like the refcount
update cost is significant too.
> I am considering implementing a new disk image format that supports
> base images, snapshots (of the guest state), clones (of the disk
> content); that has a radically simpler design & code base; that is
> always consistent "on disk"; that is friendly to delta diffing (ie.
> space-efficient when used with ZFS snapshots or rsync); and that makes
> use of checksumming & replication to detect & fix corruption of
> critical data structures (ideally this should be implemented by the
> filesystem, unfortunately ZFS is not available everywhere :D).
You have just described a high quality modern filesystem or database
engine; both would certainly be far more complex than qcow2's code.
Especially with checksumming and replication :)
ZFS isn't everywhere, but it looks like everyone wants to clone ZFS's
best features everywhere (but not it's worst feature: lots of memory
required).
I've had similar thoughts myself, by the way :-)
> I believe the key to achieve these (seemingly utopian) goals is to
> represent a disk "image" as a set of sparse files, 1 per
> snapshot/clone.
You can already do this, if your filesystem supports snapshotting. On
Linux hosts, any filesystem can snapshot by using LVM underneath it
(although it's not pretty to do). A few experimental Linux
filesystems let you snapshot at the filesystem level.
A feature you missed in the utopian vision is sharing backing store
for equal parts of files between different snapshots _after_ they've
been written in separate branches (with the same data), and also among
different VMs. It's becoming stylish to put similarity detection in
the filesystem somewhere too :-)
-- Jamie
- Re: [Qemu-devel] qcow2 corruption observed, fixed by reverting old change, (continued)
Message not available
- [Qemu-devel] Re: qcow2 corruption observed, fixed by reverting old change, Kevin Wolf, 2009/02/13
- Re: [Qemu-devel] Re: qcow2 corruption observed, fixed by reverting old change, Jamie Lokier, 2009/02/13
- Re: [Qemu-devel] Re: qcow2 corruption observed, fixed by reverting old change, Chris Wright, 2009/02/13
- Re: [Qemu-devel] Re: qcow2 corruption observed, fixed by reverting old change, Marc Bevand, 2009/02/14
- Re: [Qemu-devel] Re: qcow2 corruption observed, fixed by reverting old change, Dor Laor, 2009/02/14
- Re: [Qemu-devel] Re: qcow2 corruption observed, fixed by reverting old change, Jamie Lokier, 2009/02/14
- Re: [Qemu-devel] Re: qcow2 corruption observed, fixed by reverting old change, Marc Bevand, 2009/02/15
Re: [Qemu-devel] Re: qcow2 corruption observed, fixed by reverting old change,
Jamie Lokier <=
[Qemu-devel] Re: qcow2 corruption observed, fixed by reverting old change, Gleb Natapov, 2009/02/15
[Qemu-devel] Re: qcow2 corruption observed, fixed by reverting old change, Marc Bevand, 2009/02/15
[Qemu-devel] Re: qcow2 corruption observed, fixed by reverting old change, Marc Bevand, 2009/02/15