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
