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Re: [Qemu-devel] [partial RFC 0/2] QCow2 deduplication write mechanism
From: |
Benoît Canet |
Subject: |
Re: [Qemu-devel] [partial RFC 0/2] QCow2 deduplication write mechanism |
Date: |
Thu, 27 Sep 2012 19:11:58 +0200 |
User-agent: |
Mutt/1.5.21 (2010-09-15) |
> 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