Re: [Qemu-devel] [PATCH 02/10] qcow2: add qcow2_co_write_compressed

[Denis V. Lunev]

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