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.