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Re: [Qemu-devel] [PATCH v12 03/14] qcow2: Optimize bdrv_make_empty()
From: |
Eric Blake |
Subject: |
Re: [Qemu-devel] [PATCH v12 03/14] qcow2: Optimize bdrv_make_empty() |
Date: |
Fri, 10 Oct 2014 06:32:35 -0600 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.1.1 |
On 08/26/2014 03:36 PM, Max Reitz wrote:
> 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 emptying the L1 and
> refcount table (while having the dirty flag set) and creating a trivial
> refcount structure.
>
> If there are snapshots, fall back to the simple implementation (discard
> all clusters).
>
> Signed-off-by: Max Reitz <address@hidden>
> ---
> block/blkdebug.c | 2 +
> block/qcow2.c | 137
> ++++++++++++++++++++++++++++++++++++++++++++------
> include/block/block.h | 2 +
> 3 files changed, 126 insertions(+), 15 deletions(-)
>
> + } else {
> + int l1_clusters;
> + int64_t offset;
> + uint64_t *new_reftable;
> + uint8_t l1_ofs_rt_ofs_cls[20]; /* L1 offset; RT offset and clusters
> */
> + uint64_t rt_entry;
> +
> + ret = qcow2_cache_empty(bs, s->l2_table_cache);
> if (ret < 0) {
> - break;
> + return ret;
> + }
> +
> + ret = qcow2_cache_empty(bs, s->refcount_block_cache);
> + if (ret < 0) {
> + return ret;
> + }
> +
> + /* Refcounts will be broken utterly */
> + ret = qcow2_mark_dirty(bs);
qcow2_mark_dirty does assert(s->qcow_version >= 3). But this code can
be reached when committing a qcow2 0.10 compat-level file, right?
> + /* "Create" an empty reftable (one cluster) directly after the image
> + * header and an empty L1 table three clusters after the image
> header;
> + * the cluster between those two will be used as the first refblock
> */
> + cpu_to_be64w((uint64_t *)&l1_ofs_rt_ofs_cls[ 0], 3 *
> s->cluster_size);
> + cpu_to_be64w((uint64_t *)&l1_ofs_rt_ofs_cls[ 8],
> s->cluster_size);
> + cpu_to_be32w((uint32_t *)&l1_ofs_rt_ofs_cls[16], 1);
The offsets here feel a bit magic.
--
Eric Blake eblake redhat com +1-919-301-3266
Libvirt virtualization library http://libvirt.org
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- Re: [Qemu-devel] [PATCH v12 03/14] qcow2: Optimize bdrv_make_empty(),
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