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Re: [Qemu-devel] [RFC V4 01/30] qcow2: Add deduplication to the qcow2 sp
From: |
Benoît Canet |
Subject: |
Re: [Qemu-devel] [RFC V4 01/30] qcow2: Add deduplication to the qcow2 specification. |
Date: |
Fri, 4 Jan 2013 15:49:14 +0100 |
User-agent: |
Mutt/1.5.21 (2010-09-15) |
> > + L2 tables size 64KB is different from
> > + cluster size 4KB.
>
> Umm, doesn't the cluster_bits (bytes 20-23 of the header) determine the
> size of a cluster, rather than assuming a cluster is always 4KB? And
> later on, the spec says that "L2 tables are exactly one cluster in
> size.", so I'm not sure what this comment is doing here. Or are you
> stating that deduplication _also_ has an L2 table, which is fixed in
> size (unlike the normal L2 table for actual data)?
As most filesystems (ntfs, ext2/3/4, xfs) use 4KB blocs the deduplication
works very well with 4KB clusters.
The problem with 4KB cluster is that L2 table allocations are done very often
and require a flush to disk which kill performance.
So my patchset break compatibility with regular qcow2 image by using 64KB L2
table which are of a different size than the 4KB cluster.
I'll let the choice for the user to choose the cluster size but will default
to 4KB when creating a deduplicated image.
- [Qemu-devel] [RFC V4 00/30] QCOW2 deduplication, Benoît Canet, 2013/01/02
- [Qemu-devel] [RFC V4 02/30] qcow2: Add deduplication structures and fields., Benoît Canet, 2013/01/02
- [Qemu-devel] [RFC V4 04/30] qcow2: Make update_refcount public., Benoît Canet, 2013/01/02
- [Qemu-devel] [RFC V4 05/30] qcow2: Create a way to link to l2 tables when deduplicating., Benoît Canet, 2013/01/02
- [Qemu-devel] [RFC V4 03/30] qcow2: Add qcow2_de dup_read_missing_and_concatenate, Benoît Canet, 2013/01/02
- [Qemu-devel] [RFC V4 07/30] qcow2: Add qcow2_dedup_store_new_hashes., Benoît Canet, 2013/01/02
- [Qemu-devel] [RFC V4 08/30] qcow2: Implement qcow2_compute_cluster_hash., Benoît Canet, 2013/01/02
- [Qemu-devel] [RFC V4 09/30] qcow2: Extract qcow2_dedup_grow_table, Benoît Canet, 2013/01/02
- [Qemu-devel] [RFC V4 10/30] qcow2: Add qcow2_dedup_grow_table and use it., Benoît Canet, 2013/01/02