[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
Re: [PATCH v8 3/3] docs: qcow2: introduce compression type feature
From: |
Vladimir Sementsov-Ogievskiy |
Subject: |
Re: [PATCH v8 3/3] docs: qcow2: introduce compression type feature |
Date: |
Fri, 18 Oct 2019 14:38:34 +0000 |
18.10.2019 17:04, Eric Blake wrote:
> On 10/18/19 4:47 AM, Vladimir Sementsov-Ogievskiy wrote:
>> The patch add new additional field to 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 b971e59b1a..4eabd81363 100644
>> --- a/docs/interop/qcow2.txt
>> +++ b/docs/interop/qcow2.txt
>> @@ -109,6 +109,12 @@ least next five fields, up to the @header_length field.
>> 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,
>> + non-default compression is used for
>> compressed
>> + clusters. In this case, @header_length must
>> + be at least 105 and @compression_type field
>> + must be non-zero.
>
> I want to insist on 8-byte alignment, so this should be at least 112.
>
> Also, as pointed out in v7, use of the decoration '@header_length' and
> '@compression_type' is not consistent with the rest of the document. Drop the
> @.
Missed this, sorry(
>
>> +
>> Bits 3-63: Reserved (set to 0)
>> 80 - 87: compatible_features
>> @@ -183,7 +189,18 @@ It's allowed for the header end to cut some field in
>> the middle (in this case
>> the field is considered as absent), but in this case the part of the field
>> which is covered by @header_length must be zeroed.
>> - < ... No additional fields in the header currently ... >
>> + 104: compression_type
>> + Defines the compression method used for compressed
>> clusters.
>> + A single compression type is applied to all compressed
>> image
>> + clusters.
>> + If incompatible compression type bit is set: the field
>> must
>> + exist (i.e. @header_length >= 105) and must be non-zero
>> (
>> + which means non-zlib compression type)
>> + If incompatible compression type bit is unset: the field
>> + may not exist (if @header_length < 105) or it must be
>> zero
>> + (which means zlib).
>> + Available compression type values:
>> + 0: zlib <https://www.zlib.net/>
>
> One byte for the field is fine, but I'd still explicitly document 7 bytes of
> unused padding, since I want to insist on an 8-byte multiple.
>
OK
--
Best regards,
Vladimir