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Re: [PATCH v10 2/2] docs: qcow2: introduce compression type feature

From: Eric Blake
Subject: Re: [PATCH v10 2/2] docs: qcow2: introduce compression type feature
Date: Mon, 20 Jan 2020 13:46:29 -0600
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.3.1

On 1/20/20 11:13 AM, Vladimir Sementsov-Ogievskiy wrote:
The patch add new additional field to qcow2 header: compression_type,

s/add/adds a/
s/to/to the/

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.

It would be nice to have that patch as part of the same series, but it has already been posted to the list separately, so I'm okay with this series as just doc word-smithing while we get that patch series in soon.

Suggested-by: Denis Plotnikov <address@hidden>
Signed-off-by: Vladimir Sementsov-Ogievskiy <address@hidden>
  docs/interop/qcow2.txt | 16 +++++++++++++++-
  1 file changed, 15 insertions(+), 1 deletion(-)

diff --git a/docs/interop/qcow2.txt b/docs/interop/qcow2.txt
index 355925c35e..4569f0dba3 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 
                                  be present if this bit is set.
+ Bit 3: Compression type bit. If this bit is set,
+                                a non-default compression is used for 
+                                clusters. The compression_type field must be
+                                present and not zero.

Why must the compression_type field be non-zero? If this bit is set, an older qemu cannot use the image, regardless of the contents of the compression_type field, so for maximum back-compat, a sane app will never set this bit when compression_type is zero. But there is nothing technically wrong with setting this bit even when compression_type is 0, and newer qemu would still manage to use the image correctly with zlib compression if it were not for this arbitrary restriction.

                      Bits 3-63:  Reserved (set to 0)
80 - 87: compatible_features
@@ -189,7 +194,16 @@ present*, if not altered by a specific incompatible bit.
  *. A field is considered not present when header_length is less or equal to 
  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 
+                    All compressed clusters in an image use the same 
+                    type.
+                    If the incompatible bit "Compression type" is set: the 

Ragged edge formatting looks awkward. Either this is one paragraph ("type. If") or there should be a blank line to make it obvious there are two paragraphs.

+                    must be present and non-zero (which means non-zlib
+                    compression type). Otherwise, this field must not be 
+                    or must be zero (which means zlib).
+                    Available compression type values:
+                        0: zlib <https://www.zlib.net/>

I'm still not sure I agree with the mandate that the field must be non-zero when the incompatible feature bit is set.

Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

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