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Re: [Qemu-devel] [PATCH v8 03/14] qcow2: Optimize bdrv_make_empty()
From: |
Kevin Wolf |
Subject: |
Re: [Qemu-devel] [PATCH v8 03/14] qcow2: Optimize bdrv_make_empty() |
Date: |
Mon, 30 Jun 2014 13:33:39 +0200 |
User-agent: |
Mutt/1.5.21 (2010-09-15) |
Am 07.06.2014 um 20:51 hat Max Reitz geschrieben:
> bdrv_make_empty() is currently only called if the current image
> represents an external snapshot that has been committed to its base
> image; it is therefore unlikely to have internal snapshots. In this
> case, bdrv_make_empty() can be greatly sped up by creating an empty L1
> table and dropping all data clusters at once by recreating the refcount
> structure accordingly instead of normally discarding all clusters.
>
> If there are snapshots, fall back to the simple implementation (discard
> all clusters).
>
> Signed-off-by: Max Reitz <address@hidden>
> Reviewed-by: Eric Blake <address@hidden>
This approach looks a bit too complicated to me, and calulating the
required metadata size seems error-prone.
How about this:
1. Set the dirty flag in the header so we can mess with the L1 table
without keeping the refcounts consistent
2. Overwrite the L1 table with zeros
3. Overwrite the first n clusters after the header with zeros
(n = 2 + l1_clusters).
4. Update the header:
refcount_table_offset = cluster_size
refcount_table_clusters = 1
l1_table_offset = 3 * cluster_size
6. bdrv_truncate to n + 1 clusters
7. Now update the first 8 bytes at cluster_size (the first new refcount
table entry) to point to 2 * cluster_size (new refcount block)
8. Reset refcount block and L2 cache
9. Allocate n + 1 clusters (the header, too) and make sure you get
offset 0
10. Remove the dirty flag
Surprisingly (or not) this is much like an ordinary image creation. The
main difference is that we keep the full size of the L1 table so the
image stays always valid (the spec would even allow us to temporarily
set l1_size = 0, but qcow2_open() doesn't seem to like that) and all
areas where the L1 table could be are zeroed (this includes the new
refcount table/block until the header is updated).
I wanted to check whether this would still give the preallocation=full
series what it needs, but a v11 doesn't seem to be on the list yet and
v10 doesn't have the dependency on this series yet.
Kevin
- [Qemu-devel] [PATCH v8 00/14] qemu-img: Implement commit like QMP, Max Reitz, 2014/06/07
- [Qemu-devel] [PATCH v8 02/14] qcow2: Implement bdrv_make_empty(), Max Reitz, 2014/06/07
- [Qemu-devel] [PATCH v8 01/14] qcow2: Allow "full" discard, Max Reitz, 2014/06/07
- [Qemu-devel] [PATCH v8 03/14] qcow2: Optimize bdrv_make_empty(), Max Reitz, 2014/06/07
- Re: [Qemu-devel] [PATCH v8 03/14] qcow2: Optimize bdrv_make_empty(),
Kevin Wolf <=
- [Qemu-devel] [PATCH v8 05/14] blockjob: Add "ready" field, Max Reitz, 2014/06/07
- [Qemu-devel] [PATCH v8 04/14] blockjob: Introduce block_job_complete_sync(), Max Reitz, 2014/06/07
- [Qemu-devel] [PATCH v8 06/14] block/mirror: Improve progress report, Max Reitz, 2014/06/07
- [Qemu-devel] [PATCH v8 07/14] qemu-img: Implement commit like QMP, Max Reitz, 2014/06/07
- [Qemu-devel] [PATCH v8 08/14] qemu-img: Empty image after commit, Max Reitz, 2014/06/07
- [Qemu-devel] [PATCH v8 09/14] qemu-img: Enable progress output for commit, Max Reitz, 2014/06/07
- [Qemu-devel] [PATCH v8 10/14] qemu-img: Specify backing file for commit, Max Reitz, 2014/06/07