Re: [Qemu-devel] qcow2: slow internal snapshot creation

[Kevin Wolf]

Am 23.11.2012 15:03, schrieb Alexandre DERUMIER:
> performance is also reduced when snapshot exist. (like if they are no 
> preallocated metadatas)

Preallocation doesn't matter much these days.

> see initial git commit
> 
> http://git.qemu.org/?p=qemu.git;a=commit;h=a35e1c177debb01240243bd656caca302410d38c
> "qcow2: Metadata preallocation
> 
> This introduces a qemu-img create option for qcow2 which allows the metadata 
> to
> be preallocated, i.e. clusters are reserved in the refcount table and L1/L2
> tables, but no data is written to them. Metadata is quite small, so this
> happens in almost no time.
> 
> Especially with qcow2 on virtio this helps to gain a bit of performance during
> the initial writes. However, as soon as create a snapshot, we're back to the
> normal slow speed, obviously. So this isn't the real fix, but kind of a cheat
> while we're still having trouble with qcow2 on virtio."

This is a commit message from 2009 that already says that this wasn't
the final solution.

Kevin