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From: "Denis V. Lunev" <den@openvz.org>
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Date: Wed, 1 Jun 2016 00:00:45 +0300
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Subject: Re: [Qemu-devel] [PATCH 02/10] qcow2: add qcow2_co_write_compressed
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To: Eric Blake <eblake@redhat.com>, Pavel Butsykin <pbutsykin@virtuozzo.com>, Stefan Hajnoczi <stefanha@gmail.com>
Cc: Kevin Wolf <kwolf@redhat.com>, Jeff Cody <jcody@redhat.com>, qemu-devel@nongnu.org, Markus Armbruster <armbru@redhat.com>, Stefan Hajnoczi <stefanha@redhat.com>, John Snow <jsnow@redhat.com>

On 05/31/2016 09:42 PM, Eric Blake wrote:
> On 05/30/2016 06:58 AM, Pavel Butsykin wrote:
>
>> Sorry, but it seems this will never happen, because the second write
>> will not pass this check:
>>
>> uint64_t qcow2_alloc_compressed_cluster_offset(BlockDriverState *bs,
>>                                                 uint64_t offset,
>>                                                 int compressed_size)
>> {
>>      ...
>>      /* Compression can't overwrite anything. Fail if the cluster was
>> already
>>       * allocated. */
>>      cluster_offset = be64_to_cpu(l2_table[l2_index]);
>>      if (cluster_offset & L2E_OFFSET_MASK) {
>>          qcow2_cache_put(bs, s->l2_table_cache, (void**) &l2_table);
>>          return 0;
>>      }
>>     ...
>>
>> As you can see we can't do the compressed write in the already allocated
>> cluster.
> Umm, doesn't that defeat the point of compression, if every compressed
> cluster becomes the head of a new cluster?  The whole goal of
> compression is to be able to fit multiple clusters within one.
>
AFAIK the file will be sparse in that unused areas