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[Qemu-devel] [PATCH] qcow2: Clarify that compressed cluster offset requi
From: |
Eric Blake |
Subject: |
[Qemu-devel] [PATCH] qcow2: Clarify that compressed cluster offset requires shift |
Date: |
Mon, 29 Feb 2016 08:11:54 -0700 |
The specs for the host cluster offset of a compressed cluster
were not clear that the offset is in terms of sectors, and requires
a shift by 9 to be a byte offset. Add some more text to make the
interpretation obvious.
CC: address@hidden
Signed-off-by: Eric Blake <address@hidden>
---
docs/specs/qcow2.txt | 9 +++++++--
1 file changed, 7 insertions(+), 2 deletions(-)
diff --git a/docs/specs/qcow2.txt b/docs/specs/qcow2.txt
index 80cdfd0..7049415 100644
--- a/docs/specs/qcow2.txt
+++ b/docs/specs/qcow2.txt
@@ -323,11 +323,16 @@ 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!
+ Bit 0 - x: Bits 9-(x+9) of host cluster offset. This is
+ usually _not_ aligned to a cluster boundary!
x+1 - 61: Compressed size of the images in sectors of 512 bytes
+The bits of the host cluster offset not specified in the cluster descriptor
+are 0 (bits 0-8 are obvious because a 512-byte sector is the smallest
+addressable unit, while bits 56-63 implies that a qcow2 file cannot exceed
+2^56 bytes in size).
+
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
no backing file or the backing file is smaller than the image, they shall read
--
2.5.0
- [Qemu-devel] [PATCH] qcow2: Clarify that compressed cluster offset requires shift,
Eric Blake <=