[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
[Qemu-devel] [PULL 24/37] specs/qcow2: Fix documentation of the compress
From: |
Kevin Wolf |
Subject: |
[Qemu-devel] [PULL 24/37] specs/qcow2: Fix documentation of the compressed cluster descriptor |
Date: |
Fri, 2 Mar 2018 19:54:35 +0100 |
From: Alberto Garcia <address@hidden>
This patch fixes several mistakes in the documentation of the
compressed cluster descriptor:
1) 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* or, in other
words, the number of additional sectors after the first one.
2) the width of the fields is incorrectly specified. The number of bits
used by each field is
x = 62 - (cluster_bits - 8) for the offset field
y = (cluster_bits - 8) for the size field
So the offset field's location is [0, x-1], not [0, x] as stated.
3) the size field does not contain the size of the compressed data,
but rather the number of sectors where that data is stored. The
compressed data starts at the exact point specified in the offset
field and ends when there's enough data to produce a cluster of
decompressed data. Both points can be in the middle of a sector,
allowing several compressed clusters to be stored next to one
another, sharing sectors if necessary.
Cc: address@hidden
Signed-off-by: Alberto Garcia <address@hidden>
Reviewed-by: Eric Blake <address@hidden>
Signed-off-by: Kevin Wolf <address@hidden>
---
docs/interop/qcow2.txt | 16 +++++++++++++---
1 file changed, 13 insertions(+), 3 deletions(-)
diff --git a/docs/interop/qcow2.txt b/docs/interop/qcow2.txt
index d7fdb1fee3..feb711fb6a 100644
--- a/docs/interop/qcow2.txt
+++ b/docs/interop/qcow2.txt
@@ -426,10 +426,20 @@ 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-1: Host cluster offset. This is usually _not_ aligned to a
+ cluster or sector boundary!
- x+1 - 61: Compressed size of the images in sectors of 512 bytes
+ x - 61: Number of additional 512-byte sectors used for the
+ compressed data, beyond the sector containing the offset
+ in the previous field. Some of these sectors may reside
+ in the next contiguous host cluster.
+
+ Note that the compressed data does not necessarily occupy
+ all of the bytes in the final sector; rather, decompression
+ stops when it has produced a cluster of data.
+
+ Another compressed cluster may map to the tail of the final
+ sector used by this compressed cluster.
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
--
2.13.6
- [Qemu-devel] [PULL 16/37] vdi: Avoid bitrot of debugging code, (continued)
- [Qemu-devel] [PULL 16/37] vdi: Avoid bitrot of debugging code, Kevin Wolf, 2018/03/02
- [Qemu-devel] [PULL 17/37] vdi: Switch to .bdrv_co_block_status(), Kevin Wolf, 2018/03/02
- [Qemu-devel] [PULL 18/37] vmdk: Switch to .bdrv_co_block_status(), Kevin Wolf, 2018/03/02
- [Qemu-devel] [PULL 19/37] vpc: Switch to .bdrv_co_block_status(), Kevin Wolf, 2018/03/02
- [Qemu-devel] [PULL 20/37] vvfat: Switch to .bdrv_co_block_status(), Kevin Wolf, 2018/03/02
- [Qemu-devel] [PULL 22/37] block: fix write with zero flag set and iovector provided, Kevin Wolf, 2018/03/02
- [Qemu-devel] [PULL 21/37] block: Drop unused .bdrv_co_get_block_status(), Kevin Wolf, 2018/03/02
- [Qemu-devel] [PULL 23/37] iotest 033: add misaligned write-zeroes test via truncate, Kevin Wolf, 2018/03/02
- [Qemu-devel] [PULL 26/37] aio: rename aio_context_in_iothread() to in_aio_context_home_thread(), Kevin Wolf, 2018/03/02
- [Qemu-devel] [PULL 25/37] docs: document how to use the l2-cache-entry-size parameter, Kevin Wolf, 2018/03/02
- [Qemu-devel] [PULL 24/37] specs/qcow2: Fix documentation of the compressed cluster descriptor,
Kevin Wolf <=
- [Qemu-devel] [PULL 28/37] block: add BlockBackend->in_flight counter, Kevin Wolf, 2018/03/02
- [Qemu-devel] [PULL 27/37] block: extract AIO_WAIT_WHILE() from BlockDriverState, Kevin Wolf, 2018/03/02
- [Qemu-devel] [PULL 30/37] Revert "IDE: Do not flush empty CDROM drives", Kevin Wolf, 2018/03/02
- [Qemu-devel] [PULL 29/37] block: test blk_aio_flush() with blk->root == NULL, Kevin Wolf, 2018/03/02
- [Qemu-devel] [PULL 32/37] qcow2: make qcow2_co_create2() a coroutine_fn, Kevin Wolf, 2018/03/02
- [Qemu-devel] [PULL 33/37] qemu-img: Make resize error message more general, Kevin Wolf, 2018/03/02
- [Qemu-devel] [PULL 31/37] block: rename .bdrv_create() to .bdrv_co_create_opts(), Kevin Wolf, 2018/03/02
- [Qemu-devel] [PULL 35/37] block/ssh: Make ssh_grow_file() blocking, Kevin Wolf, 2018/03/02
- [Qemu-devel] [PULL 34/37] block/ssh: Pull ssh_grow_file() from ssh_create(), Kevin Wolf, 2018/03/02