The documentation claims that the cluster descriptor contains the number of sectors used to store the compressed data, but what it actually contains is the number of sectors *minus one*.
That can be easily seen in qcow2_decompress_cluster(), that adds one to the value stored in that field: nb_csectors = ((cluster_offset >> s->csize_shift) & s->csize_mask) + 1; In addition to that this patch clarifies where the actual compressed data is located. Although the size of the data is specified in sectors, the offset is not necessarily aligned to a sector boundary, so the actual data goes from the specified offset until the end of the last sector, leaving the initial bytes of the first sector (if any) unused. Signed-off-by: Alberto Garcia <be...@igalia.com> --- v2: I realized that the documentation is not completely clear about the exact location and size of the compressed data, so I updated the patch to clarify this. --- docs/interop/qcow2.txt | 12 ++++++++++-- 1 file changed, 10 insertions(+), 2 deletions(-) diff --git a/docs/interop/qcow2.txt b/docs/interop/qcow2.txt index d7fdb1fee3..dc2b9cefb2 100644 --- a/docs/interop/qcow2.txt +++ b/docs/interop/qcow2.txt @@ -427,9 +427,17 @@ Standard Cluster Descriptor: Compressed Clusters Descriptor (x = 62 - (cluster_bits - 8)): Bit 0 - x: Host cluster offset. This is usually _not_ aligned to a - cluster boundary! + cluster or sector boundary! - x+1 - 61: Compressed size of the images in sectors of 512 bytes + x+1 - 61: Number of 512-byte sectors used for the compressed data, + minus one (that is, a value of n here means n+1 sectors). + + The actual compressed data is located at the end of this + region, from the offset indicated in the previous field + until the end of the last sector. + + The initial bytes of this region are therefore unused if + the offset is not aligned to a sector boundary. If a cluster is unallocated, read requests shall read the data from the backing file (except if bit 0 in the Standard Cluster Descriptor is set). If there is -- 2.11.0