Re: [Qemu-devel] [partial RFC 0/2] QCow2 deduplication write mechanism

["Benoît Canet" <benoit.canet@irqsave.net>]

> If I understood correctly, this does cluster-level dedup within a qcow2
> image.

yes

> What is the motivation here?  Reduce space usage if a guest copies files
> internally?
First use case is people using LXC, OpenVZ or chroot inside vms they rent
to their "cloud" provider.

Second use case is applications generating duplicates blocks.
A well known CAD software does this and on a filer doing deduplication
on 4KB blocks the dedup ratio is around five.

> 
> Why use cluster granularity?  If the guest uses smaller granularity, it
> will misalign the data wrt cluster boundaries, and deduplication will fail.

Using cluster granularity would allow to use the qcow2 refcount and l1/l2
tables to track duplicated blocks.
The patch embryo does read the missing data if given data is misaligned.
To avoid hurting performance using a smaller cluster size with the adequate
changes in qcow2 may help.

Regards

Benoît