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Re: [Qemu-devel] [RFC PATCH 01/16] Specification for qcow2 version 3
From: |
Eric Blake |
Subject: |
Re: [Qemu-devel] [RFC PATCH 01/16] Specification for qcow2 version 3 |
Date: |
Tue, 27 Mar 2012 10:25:14 -0600 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:11.0) Gecko/20120316 Thunderbird/11.0 |
On 03/27/2012 09:03 AM, Kevin Wolf wrote:
> This is the second draft for what I think could be added when we increase
> qcow2's
> version number to 3. This includes points that have been made by several
> people
> over the past few months. We're probably not going to implement this next
> week,
> but I think it's important to get discussions started early, so here it is.
>
> +If the version is 3 or higher, the header has the following additional
> fields.
> +For version 2, the values are assumed to be zero, unless specified otherwise
> +in the description of a field.
> +
> + 72 - 79: incompatible_features
> + Bitmask of incompatible features. An implementation must
> + fail to open an image if an unknown bit is set.
> +
> + Bit 0: The reference counts in the image file may be
> + inaccurate. Implementations must
> check/rebuild
> + them if they rely on them.
> +
> + Bit 1: Enable subclusters. This affects the L2 table
> + format.
> +
> + Bits 2-31: Reserved (set to 0)
Offsets 72-79 forms 8 bytes, so this should be bits 2-63 are reserved.
> +
> + 80 - 87: compatible_features
> + Bitmask of compatible features. An implementation can
> + safely ignore any unknown bits that are set.
> +
> + Bits 0-31: Reserved (set to 0)
Again, bits 0-63, based on offsets.
> +
> + 88 - 95: autoclear_features
> + Bitmask of auto-clear features. An implementation may
> only
> + write to an image with unknown auto-clear features if it
> + clears the respective bits from this field first.
> +
> + Bits 0-31: Reserved (set to 0)
And again.
> +
> + 96 - 99: refcount_bits
> + Size of a reference count block entry in bits. For
> version 2
> + images, the size is always assumed to be 16 bits. The
> size
> + must be a power of two.
> + [ TODO: Define order in sub-byte sizes ]
> +
> + 100 - 103: header_length
> + Length of the header structure in bytes. For version 2
> + images, the length is always assumed to be 72 bytes.
Might be a good idea to require this to be a multiple of 8, since both
72 and 104 qualify, and since header extensions are also required to be
padded out to multiples of 8.
> +== Feature name table ==
> +
> +A feature name table is an optional header extension that contains the name
> for
> +features used by the image. It can be used by applications that don't know
> +the respective feature (e.g. because the feature was introduced only later)
> to
> +display a useful error message.
> +
> +The number of entries in the feature name table is determined by the length
> of
> +the header extension data. Its entries look like this:
> +
> + Byte 0: Type of feature (select feature bitmap)
> + 0: Incompatible feature
> + 1: Compatible feature
> + 2: Autoclear feature
> +
> + 1: Bit number within the selected feature bitmap
> +
> + 2 - 47: Feature name (padded with zeros, but not necessarily null
> + terminated if it has full length)
Semantic nit: The NUL character is all zeros; it is one byte in all
unibyte and multi-byte encodings, and the NUL wide character is the
all-zero wchar_t value; while 'null' refers to a pointer to nowhere.
Saying a string is null terminated is wrong, because you don't have a 4-
or 8-byte NULL pointer at the end of the string, just a one-byte NUL
character. Therefore, strings are nul-terminated, not null-terminated.
Is this extension capped at 48 bytes, or it is a repeating table of as
many 48-byte multiples as necessary to represent each feature name?
--
Eric Blake address@hidden +1-919-301-3266
Libvirt virtualization library http://libvirt.org
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- [Qemu-devel] [RFC PATCH 00/16] qcow2: Basic version 3 support, Kevin Wolf, 2012/03/27
- [Qemu-devel] [RFC PATCH 08/16] qcow2: Ignore reserved bits in refcount table entries, Kevin Wolf, 2012/03/27
- [Qemu-devel] [RFC PATCH 03/16] qcow2: Ignore reserved bits in count_contiguous_clusters(), Kevin Wolf, 2012/03/27
- [Qemu-devel] [RFC PATCH 05/16] qcow2: Ignore reserved bits in L1/L2 entries, Kevin Wolf, 2012/03/27
- [Qemu-devel] [RFC PATCH 12/16] qcow2: Support for feature table header extension, Kevin Wolf, 2012/03/27
- [Qemu-devel] [RFC PATCH 01/16] Specification for qcow2 version 3, Kevin Wolf, 2012/03/27
- Re: [Qemu-devel] [RFC PATCH 01/16] Specification for qcow2 version 3,
Eric Blake <=
- [Qemu-devel] [RFC PATCH 04/16] qcow2: Fail write_compressed when overwriting data, Kevin Wolf, 2012/03/27
- [Qemu-devel] [RFC PATCH 06/16] qcow2: Refactor qcow2_free_any_clusters, Kevin Wolf, 2012/03/27
- [Qemu-devel] [RFC PATCH 09/16] qcow2: Ignore reserved bits in check_refcounts, Kevin Wolf, 2012/03/27
- [Qemu-devel] [RFC PATCH 02/16] qcow2: Ignore reserved bits in get_cluster_offset, Kevin Wolf, 2012/03/27
- [Qemu-devel] [RFC PATCH 14/16] qemu-iotests: Test COW with zero clusters, Kevin Wolf, 2012/03/27
- [Qemu-devel] [RFC PATCH 11/16] qcow2: Support reading zero clusters, Kevin Wolf, 2012/03/27
- [Qemu-devel] [RFC PATCH 13/16] qemu-iotests: add a simple test for write_zeroes, Kevin Wolf, 2012/03/27
- [Qemu-devel] [RFC PATCH 15/16] qcow2: Zero write support, Kevin Wolf, 2012/03/27
- [Qemu-devel] [RFC PATCH 16/16] qemu-iotests: use qcow3, Kevin Wolf, 2012/03/27
- [Qemu-devel] [RFC PATCH 10/16] qcow2: Version 3 images, Kevin Wolf, 2012/03/27