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| author | Vladimir Sementsov-Ogievskiy | 2020-01-31 15:22:19 +0100 |
|---|---|---|
| committer | Max Reitz | 2020-02-20 16:42:52 +0100 |
| commit | 66fcbca5a7202d2ff2973b61a30f915204917eb7 (patch) | |
| tree | 771bbf9f9d44016127c72574193f7baf2b92cac5 | |
| parent | docs: improve qcow2 spec about extending image header (diff) | |
| download | qemu-66fcbca5a7202d2ff2973b61a30f915204917eb7.tar.gz qemu-66fcbca5a7202d2ff2973b61a30f915204917eb7.tar.xz qemu-66fcbca5a7202d2ff2973b61a30f915204917eb7.zip | |
docs: qcow2: introduce compression type feature
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>
| -rw-r--r-- | docs/interop/qcow2.txt | 21 |
1 files changed, 19 insertions, 2 deletions
diff --git a/docs/interop/qcow2.txt b/docs/interop/qcow2.txt index 823cc266e0..5597e24474 100644 --- a/docs/interop/qcow2.txt +++ b/docs/interop/qcow2.txt @@ -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 === |