[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
Re: [Qemu-block] [PATCH V5 01/10] specs/qcow2: add compress format exten
Re: [Qemu-block] [PATCH V5 01/10] specs/qcow2: add compress format extension
Wed, 13 Dec 2017 17:43:29 +0100
Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.2.1
Am 18.09.2017 um 12:50 schrieb Kevin Wolf:
> Am 18.09.2017 um 12:09 hat Peter Lieven geschrieben:
>> Am 11.09.2017 um 16:22 schrieb Kevin Wolf:
>>> Am 25.07.2017 um 16:41 hat Peter Lieven geschrieben:
>>>> Signed-off-by: Peter Lieven <address@hidden>
>>>> docs/interop/qcow2.txt | 51
>>>> roms/ipxe | 2 +-
>>>> 2 files changed, 51 insertions(+), 2 deletions(-)
>>>> diff --git a/docs/interop/qcow2.txt b/docs/interop/qcow2.txt
>>>> index d7fdb1f..d0d2a8f 100644
>>>> --- a/docs/interop/qcow2.txt
>>>> +++ b/docs/interop/qcow2.txt
>>>> @@ -86,7 +86,12 @@ in the description of a field.
>>>> be written to (unless for regaining
>>>> - Bits 2-63: Reserved (set to 0)
>>>> + Bit 2: Compress format bit. If and only if this
>>>> + is set then the compress format extension
>>>> + MUST be present and MUST be parsed and
>>>> + for compatibility.
>>>> + Bits 3-63: Reserved (set to 0)
>>>> 80 - 87: compatible_features
>>>> Bitmask of compatible features. An implementation can
>>>> @@ -137,6 +142,7 @@ be stored. Each extension has a structure like the
>>>> 0x6803f857 - Feature name table
>>>> 0x23852875 - Bitmaps extension
>>>> 0x0537be77 - Full disk encryption header pointer
>>>> + 0xC03183A3 - Compress format extension
>>>> other - Unknown header extension, can be
>>>> @@ -311,6 +317,49 @@ The algorithms used for encryption vary depending on
>>>> the method
>>>> in the LUKS header, with the physical disk sector as the
>>>> input tweak.
>>>> +== Compress format extension ==
>>>> +The compress format extension is an optional header extension. It provides
>>>> +the ability to specify the compress algorithm and compress parameters
>>>> +that are used for compressed clusters. This new header MUST be present if
>>>> +the incompatible-feature bit "compress format bit" is set and MUST be
>>>> +The fields of the compress format extension are:
>>>> + Byte 0 - 13: compress_format_name (padded with zeros, but not
>>>> + necessarily null terminated if it has full length).
>>>> + Valid compression format names currently are:
>>>> + deflate: Standard zlib deflate compression without
>>>> + compression header
>>>> + 14: compress_level (uint8_t)
>>>> + 0 = default compress level (valid for all formats,
>>>> + Additional valid compression levels for deflate
>>>> + All values between 1 and 9. 1 gives best speed, 9
>>>> gives best
>>>> + compression. The default compression level is defined
>>>> by zlib
>>>> + and currently defaults to 6.
>>>> + 15: compress_window_size (uint8_t)
>>>> + Window or dictionary size used by the compression
>>>> + Currently only used by the deflate compression
>>>> + Valid window sizes for deflate compression range from
>>>> 8 to
>>>> + 15 inclusively.
>>> So what is the plan with respect to adding new compression algorithms?
>>> If I understand correctly, we'll use the same extension type
>>> (0xC03183A3) and a different compress_format_name. However, the other
>>> algorithm will likely have different options and also a different number
>>> of options. Will the meaning of bytes 14-end then depend on the specific
>> The idea of the current options is that almost every algorithm will have
>> a compression level setting and most have a dictionary or window
>> size. This is why I added them to the common header.
> It's this kind of assumptions that lead to awkward interfaces in the
> long run, because if you say "almost every" case looks like this, you
> can be sure that one day you'll get one of the remaining cases where the
> field becomes useless.
> Also, while most formats may support a compress_level, it is also likely
> that each of them uses a different range of values and a different
> default. So they look very similar, but are in fact different.
> I think this is best dealt with by treating these options as specific to
> the format, and if many formats coincide to have a field with the same
> name at the same place, so be it.
> If one day we finally get to a state where 'qemu-img create' options are
> expressed in the QAPI schema, I would include the fields in the
> individual branches of the union type (with a documentation what the
> exact semantics are for the specific format) rather than in the base
> type where you have to explain the semantics without actually referring
> to the branches.
>>> Part of why I'm asking this is because I wonder why compress_format_name
>>> is 14 characters. It looks to me as if that is intended to make the
>>> header a round 16 bytes for the deflate algorithm. But unless we say
>>> "16 bits ought to be enough for every algorithm", this is just
>>> optimising a special case. (Or actually not optimising, but just moving
>>> the padding from the end of the header extension to padding inside the
>>> compress_format_name field.)
>>> Wouldn't 8 bytes still be plenty of space for a format name, and look
>>> more natural? Then any format that requires options would have a little
>>> more space without immediately going to a 24 byte header extension.
>> Sure 8 characters will still be enough. I can respin the series with
>> an updated header if you like.
> Yes, I would prefer this.
Somehow, I forgot to respin this series. I will send a new version for 2.12
|[Prev in Thread]
||[Next in Thread]|
- Re: [Qemu-block] [PATCH V5 01/10] specs/qcow2: add compress format extension,
Peter Lieven <=