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docs: qcow2: introduce compression type feature

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The patch adds a new additional field to the qcow2 header: compression_type,
which specifies compression type. If field is absent or zero, default
compression type is set: ZLIB, which corresponds to current behavior.

New compression type (ZSTD) is to be added in further commit.

Suggested-by: Denis Plotnikov <dplotnikov@virtuozzo.com>
Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20200131142219.3264-3-vsementsov@virtuozzo.com>
[mreitz: s/Bits 3-63:  Reserved/Bits 4-63:  Reserved/]
Signed-off-by: Max Reitz <mreitz@redhat.com>
This commit is contained in:
Vladimir Sementsov-Ogievskiy 2020-01-31 17:22:19 +03:00 committed by Max Reitz
parent 3ae3fcfae5
commit 66fcbca5a7
1 changed files with 19 additions and 2 deletions
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21
docs/interop/qcow2.txt
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@ -109,7 +109,12 @@ the next fields through header_length.
An External Data File Name header extension may
be present if this bit is set.
Bits 3-63: Reserved (set to 0)
Bit 3: Compression type bit. If this bit is set,
a non-default compression is used for compressed
clusters. The compression_type field must be
present and not zero.
Bits 4-63: Reserved (set to 0)
80 - 87: compatible_features
Bitmask of compatible features. An implementation can
@ -190,7 +195,19 @@ present*, if not altered by a specific incompatible bit.
to the field's offset. Also, all additional fields are not present for
version 2.
< ... No additional fields in the header currently ... >
104: compression_type
Defines the compression method used for compressed clusters.
All compressed clusters in an image use the same compression
type.
If the incompatible bit "Compression type" is set: the field
must be present and non-zero (which means non-zlib
compression type). Otherwise, this field must not be present
or must be zero (which means zlib).
Available compression type values:
0: zlib <https://www.zlib.net/>
=== Header padding ===