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Re: [PATCH v11 2/2] docs: qcow2: introduce compression type feature
From: |
Eric Blake |
Subject: |
Re: [PATCH v11 2/2] docs: qcow2: introduce compression type feature |
Date: |
Fri, 31 Jan 2020 08:46:12 -0600 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.4.1 |
On 1/31/20 8:22 AM, Vladimir Sementsov-Ogievskiy wrote:
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 <address@hidden>
Signed-off-by: Vladimir Sementsov-Ogievskiy <address@hidden>
---
docs/interop/qcow2.txt | 19 ++++++++++++++++++-
1 file changed, 18 insertions(+), 1 deletion(-)
diff --git a/docs/interop/qcow2.txt b/docs/interop/qcow2.txt
index 823cc266e0..3a8d5c0c6e 100644
--- a/docs/interop/qcow2.txt
+++ b/docs/interop/qcow2.txt
@@ -109,6 +109,11 @@ the next fields through header_length.
An External Data File Name header extension
may
be present if this bit is set.
+ 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.
+
I'm still not sold on this. What's wrong with:
Bit 3: Compression type bit. If this bit is set,
a non-default compression may be used for
compressed clusters, and the compression_type
field must be present.
Bits 3-63: Reserved (set to 0)
80 - 87: compatible_features
@@ -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).
If the incompatible bit "Compression type" is set: the field
must be present. Otherwise, this field must not be present
or must be zero (which means zlib). It is recommended but
not required that the "Compression type" bit be clear if this
field is present but zero.
+
+ Available compression type values:
+ 0: zlib <https://www.zlib.net/>
=== Header padding ===
--
Eric Blake, Principal Software Engineer
Red Hat, Inc. +1-919-301-3226
Virtualization: qemu.org | libvirt.org