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Re: [Qemu-block] [PATCH V3 1/9] specs/qcow2: add compress format extensi
From: |
Eric Blake |
Subject: |
Re: [Qemu-block] [PATCH V3 1/9] specs/qcow2: add compress format extension |
Date: |
Fri, 14 Jul 2017 09:52:40 -0500 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.2.1 |
On 07/14/2017 04:56 AM, Peter Lieven wrote:
> Signed-off-by: Peter Lieven <address@hidden>
> ---
> docs/interop/qcow2.txt | 35 ++++++++++++++++++++++++++++++++++-
> 1 file changed, 34 insertions(+), 1 deletion(-)
>
> diff --git a/docs/interop/qcow2.txt b/docs/interop/qcow2.txt
> index d7fdb1f..c2d3dab 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
> consistency).
>
> - Bits 2-63: Reserved (set to 0)
> + Bit 2: Compress format bit. If and only if this bit
> + is set then the compress format extension
> + MUST be present and MUST be parsed and
> checked
> + 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
> following:
> 0x6803f857 - Feature name table
> 0x23852875 - Bitmaps extension
> 0x0537be77 - Full disk encryption header pointer
> + 0xC03183A3 - Compress format extension
> other - Unknown header extension, can be safely
> ignored
>
> @@ -311,6 +317,33 @@ 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 absent
> +otherwise.
Probably worth mentioning that omitting the incompatible "Compress
format bit" is the same as setting it and populating the Compress format
extension with "zlib"/0. Don't know if that fits better here, or up
above at the incompatible feature bit section.
> +
> +The fields of the compress format extension are:
> +
> + Byte 0 - 15: compress_format_name (padded with zeros, but not
> + necessarily null terminated if it has full length).
> + Valid compression format names currently are:
> +
> + zlib: Standard zlib deflate compression
16 bytes,
> +
> + 16: compress_level (uint8_t)
+ 1, means you now have to pad the struct. I'd make the
compress_format_name be only 15 bits (bytes 0-14, compress_level at 15),
so that you have nice alignment without wasting padding.
> +
> + 0 = default compress level (valid for all formats,
> default)
> +
> + Additional valid compression levels for zlib compression:
> +
> + All values between 1 and 9. 1 gives best speed, 9 gives
> best
> + compression.
Is there a particular value between 1 and 9 that matches the output you
get when using 0?
--
Eric Blake, Principal Software Engineer
Red Hat, Inc. +1-919-301-3266
Virtualization: qemu.org | libvirt.org
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Description: OpenPGP digital signature
- [Qemu-block] [PATCH V3 0/9] add Qcow2 compress format extension, Peter Lieven, 2017/07/14
- [Qemu-block] [PATCH V3 2/9] qapi/block-core: add Qcow2Compress parameters, Peter Lieven, 2017/07/14
- [Qemu-block] [PATCH V3 1/9] specs/qcow2: add compress format extension, Peter Lieven, 2017/07/14
- Re: [Qemu-block] [PATCH V3 1/9] specs/qcow2: add compress format extension,
Eric Blake <=
- [Qemu-block] [PATCH V3 8/9] block/qcow2: start using the compress format extension, Peter Lieven, 2017/07/14
- [Qemu-block] [PATCH V3 3/9] block/qcow2: parse compress create options, Peter Lieven, 2017/07/14
- [Qemu-block] [PATCH V3 5/9] block/qcow2: read and write the compress format extension, Peter Lieven, 2017/07/14
- [Qemu-block] [PATCH V3 9/9] block/qcow2: add lzo compress format, Peter Lieven, 2017/07/14
- [Qemu-block] [PATCH V3 7/9] block/qcow2: optimize qcow2_co_pwritev_compressed, Peter Lieven, 2017/07/14
- [Qemu-block] [PATCH V3 4/9] qemu-img: add documentation for compress settings, Peter Lieven, 2017/07/14
- [Qemu-block] [PATCH V3 6/9] block/qcow2: simplify ret usage in qcow2_create, Peter Lieven, 2017/07/14