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[Qemu-block] [PATCH v2] specs/qcow2: Fix documentation of the compressed

From: Alberto Garcia
Subject: [Qemu-block] [PATCH v2] specs/qcow2: Fix documentation of the compressed cluster descriptor
Date: Tue, 20 Feb 2018 19:01:35 +0200

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 <address@hidden>

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

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