Re: [Qemu-devel] [RFC] qed: Add QEMU Enhanced Disk format

[Anthony Liguori]

On 09/08/2010 03:23 AM, Avi Kivity wrote:
>  On 09/08/2010 01:27 AM, Anthony Liguori wrote:
> > > FWIW, L2s are 256K at the moment and with a two level table, it can 
> > > support 5PB of data.
> >
> >
> > I clearly suck at basic math today.  The image supports 64TB today.  
> > Dropping to 128K tables would reduce it to 16TB and 64k tables would 
> > be 4TB.
>
> Maybe we should do three levels then.  Some users are bound to 
> complain about 64TB.

That's just the default size.  The table size and cluster sizes are 
configurable.  Without changing the cluster size, the image can support 
up to 1PB.

> > BTW, I don't think your checksumming idea is sound.  If you store a 
> > 64-bit checksum along side each point, it becomes necessary to update 
> > the parent pointer every time the table changes.   This introduces an 
> > ordering requirement which means you need to sync() the file every 
> > time you update and L2 entry.
>
> Even worse, if the crash happens between an L2 update and an L1 
> checksum update, the entire cluster goes away.  You really want 
> allocate-on-write for this.
>
> > Today, we only need to sync() when we first allocate an L2 entry 
> > (because their locations never change).  From a performance 
> > perspective, it's the difference between an fsync() every 64k vs. 
> > every 2GB.
>
> Yup.  From a correctness perspective, it's the difference between a 
> corrupted filesystem on almost every crash and a corrupted filesystem 
> in some very rare cases.

I'm not sure I understand you're corruption comment.  Are you claiming 
that without checksumming, you'll often get corruption or are you 
claiming that without checksums, if you don't sync metadata updates 
you'll get corruption?

qed is very careful about ensuring that we don't need to do syncs and we 
don't get corruption because of data loss.  I don't necessarily buy your 
checksumming argument.

> > Plus, doesn't btrfs do block level checksumming?  IOW, if you run a 
> > workload where you care about this level of data integrity 
> > validation, if you did btrfs + qed, you would be fine.
>
> Or just btrfs by itself (use btrfs for snapshots and base images, use 
> qemu-img convert for shipping).
>
> > Since the majority of file systems don't do metadata checksumming, 
> > it's not obvious to me that we should be. 
>
> The logic is that as data sizes increase, the probablity of error 
> increases.
>
> > I think one of the critical flaws in qcow2 was trying to invent a 
> > better filesystem within qemu instead of just sticking to a very 
> > simple and obviously correct format and letting the FS folks do the 
> > really fancy stuff.
>
> Well, if we introduce a minimal format, we need to make sure it isn't 
> too minimal.
>
> I'm still not sold on the idea.  What we're doing now is pushing the 
> qcow2 complexity to users.  We don't have to worry about refcounts 
> now, but users have to worry whether they're the machine they're 
> copying the image to supports qed or not.
>
> The performance problems with qcow2 are solvable.  If we preallocate 
> clusters, the performance characteristics become essentially the same 
> as qed.

By creating two code paths within qcow2.  It's not just the reference 
counts, it's the lack of guaranteed alignment, compression, and some of 
the other poor decisions in the format.

If you have two code paths in qcow2, you have non-deterministic 
performance because users that do reasonable things with their images 
will end up getting catastrophically bad performance.

A new format doesn't introduce much additional complexity.  We provide 
image conversion tool and we can almost certainly provide an in-place 
conversion tool that makes the process very fast.

Regards,

Anthony Liguori