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Re: [Qemu-devel] [PATCH v7] spec: add qcow2 bitmaps extension specificat
From: |
John Snow |
Subject: |
Re: [Qemu-devel] [PATCH v7] spec: add qcow2 bitmaps extension specification |
Date: |
Mon, 11 Jan 2016 19:30:27 -0500 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.3.0 |
On 01/11/2016 08:05 AM, Vladimir Sementsov-Ogievskiy wrote:
> The new feature for qcow2: storing bitmaps.
>
> This patch adds new header extension to qcow2 - Bitmaps Extension. It
> provides an ability to store virtual disk related bitmaps in a qcow2
> image. For now there is only one type of such bitmaps: Dirty Tracking
> Bitmap, which just tracks virtual disk changes from some moment.
>
> Note: Only bitmaps, relative to the virtual disk, stored in qcow2 file,
> should be stored in this qcow2 file. The size of each bitmap
> (considering its granularity) is equal to virtual disk size.
>
> Signed-off-by: Vladimir Sementsov-Ogievskiy <address@hidden>
> ---
>
> v7:
>
> - Rewordings, grammar.
> Max, Eric, John, thank you very much.
>
> - add last paragraph: remaining bits in bitmap data clusters must be
> zero.
>
> - s/Bitmap Directory/bitmap directory/ and other names like this at
> the request of Max.
>
> v6:
>
> - reword bitmap_directory_size description
> - bitmap type: make 0 reserved
> - extra_data_size: resize to 4bytes
> Also, I've marked this field as "must be zero". We can always change
> it, if we decide allowing managing app to specify any extra data, by
> defining some magic value as a top of user extra data.. So, for now
> non zeor extra_data_size should be considered as an error.
> - swap name and extra_data to give good alignment to extra_data.
>
>
> v5:
>
> - 'Dirty bitmaps' renamed to 'Bitmaps', as we may have several types of
> bitmaps.
> - rewordings
> - move upper bounds to "Notes about Qemu limits"
> - s/should/must somewhere. (but not everywhere)
> - move name_size field closer to name itself in bitmap header
> - add extra data area to bitmap header
> - move bitmap data description to separate section
>
> docs/specs/qcow2.txt | 172
> ++++++++++++++++++++++++++++++++++++++++++++++++++-
> 1 file changed, 171 insertions(+), 1 deletion(-)
>
> diff --git a/docs/specs/qcow2.txt b/docs/specs/qcow2.txt
> index 121dfc8..997239d 100644
> --- a/docs/specs/qcow2.txt
> +++ b/docs/specs/qcow2.txt
> @@ -103,7 +103,18 @@ in the description of a field.
> write to an image with unknown auto-clear features if it
> clears the respective bits from this field first.
>
> - Bits 0-63: Reserved (set to 0)
> + Bit 0: Bitmaps extension bit
> + This bit indicates consistency for the
> bitmaps
> + extension data.
> +
> + It is an error if this bit is set without the
> + bitmaps extension present.
> +
> + If the bitmaps extension is present but this
> + bit is unset, the bitmaps extension data is
> + inconsistent.
> +
> + Bits 1-63: Reserved (set to 0)
>
> 96 - 99: refcount_order
> Describes the width of a reference count block entry
> (width
> @@ -123,6 +134,7 @@ be stored. Each extension has a structure like the
> following:
> 0x00000000 - End of the header extension area
> 0xE2792ACA - Backing file format name
> 0x6803f857 - Feature name table
> + 0x23852875 - Bitmaps extension
> other - Unknown header extension, can be safely
> ignored
>
> @@ -166,6 +178,34 @@ the header extension data. Each entry look like this:
> terminated if it has full length)
>
>
> +== Bitmaps extension ==
> +
> +The bitmaps extension is an optional header extension. It provides the
> ability
> +to store bitmaps related to a virtual disk. For now, there is only one bitmap
> +type: the dirty tracking bitmap, which tracks virtual disk changes from some
> +point in time.
> +
> +The data of the extension should be considered consistent only if the
> +corresponding auto-clear feature bit is set, see autoclear_features above.
> +
> +The fields of the bitmaps extension are:
> +
> + 0 - 3: nb_bitmaps
> + The number of bitmaps contained in the image. Must be
> + greater than or equal to 1.
> +
> + Note: Qemu currently only supports up to 65535 bitmaps per
> + image.
> +
> + 4 - 7: bitmap_directory_size
> + Size of the bitmap directory in bytes. It is the
> cumulative
> + size of all (nb_bitmaps) bitmap headers.
> +
> + 8 - 15: bitmap_directory_offset
> + Offset into the image file at which the bitmap directory
> + starts. Must be aligned to a cluster boundary.
> +
> +
> == Host cluster management ==
>
> qcow2 manages the allocation of host clusters by maintaining a reference
> count
> @@ -360,3 +400,133 @@ Snapshot table entry:
>
> variable: Padding to round up the snapshot table entry size to the
> next multiple of 8.
> +
> +
> +== Bitmaps ==
> +
> +As mentioned above, the bitmaps extension provides the ability to store
> bitmaps
> +related a virtual disk. This section describes how these bitmaps are stored.
> +
> +Note: all bitmaps are related to the virtual disk stored in this image.
> +
> +=== Bitmap directory ===
> +
> +Each bitmap saved in the image is described in a bitmap directory entry. The
> +bitmap directory is a contiguous area in the image file, whose starting
> offset
> +and length are given by the header extension fields bitmap_directory_offset
> and
> +bitmap_directory_size. The entries of the bitmap directory have variable
> +length, depending on the length of the bitmap name and extra data. These
> +entries are also called bitmap headers.
> +
> +Structure of a bitmap directory entry:
> +
> + Byte 0 - 7: bitmap_table_offset
> + Offset into the image file at which the bitmap table
> + (described below) for the bitmap starts. Must be aligned
> to
> + a cluster boundary.
> +
> + 8 - 11: bitmap_table_size
> + Number of entries in the bitmap table of the bitmap.
> +
> + 12 - 15: flags
> + Bit
> + 0: in_use
> + The bitmap was not saved correctly and may be
> + inconsistent.
> +
> + 1: auto
> + The bitmap must reflect all changes of the virtual
> + disk by any application that would write to this
> qcow2
> + file (including writes, snapshot switching, etc.).
> The
> + type of this bitmap must be 'dirty tracking bitmap'.
> +
> + Bits 2 - 31 are reserved and must be 0.
> +
> + 16: type
> + This field describes the sort of the bitmap.
> + Values:
> + 1: Dirty tracking bitmap
> +
> + Values 0, 2 - 255 are reserved.
> +
> + 17: granularity_bits
> + Granularity bits. Valid values: 0 - 63.
> +
> + Note: Qemu currently doesn't support granularity_bits
> + greater than 31.
> +
> + Granularity is calculated as
> + granularity = 1 << granularity_bits
> +
> + A bitmap's granularity is how many bytes of the image
> + accounts for one bit of the bitmap.
> +
> + 18 - 19: name_size
> + Size of the bitmap name. Must be non-zero.
> +
> + Note: Qemu currently doesn't support values greater than
> + 1023.
> +
> + 20 - 23: extra_data_size
> + Size of type-specific extra data.
> +
> + For now, as no extra data is defined, extra_data_size is
> + reserved and must be zero.
> +
> + variable: Type-specific extra data for the bitmap.
> +
> + variable: The name of the bitmap (not null terminated). Must be
> + unique among all bitmap names within the bitmaps
> extension.
> +
> + variable: Padding to round up the bitmap directory entry size to
> the
> + next multiple of 8.
> +
> +=== Bitmap table ===
> +
> +Bitmaps are stored using a one-level structure (as opposed to two-level
> +structure like for refcounts and guest clusters mapping) for the mapping of
> +bitmap data to host clusters. This structure is called the bitmap table.
> +
> +Each bitmap table has a variable size (stored in the bitmap directory Entry)
> +and may use multiple clusters, however, it must be contiguous in the image
> +file.
> +
> +Structure of a bitmap table entry:
> +
> + Bit 0: Reserved and must be zero if bits 9 - 55 are non-zero.
> + If bits 9 - 55 are zero:
> + 0: Cluster should be read as all zeros.
> + 1: Cluster should be read as all ones.
> +
> + 1 - 8: Reserved and must be zero.
> +
> + 9 - 55: Bits 9 - 55 of the host cluster offset. Must be aligned
> to
> + a cluster boundary. If the offset is 0, the cluster is
> + unallocated; in that case, bit 0 determines how this
> + cluster should be treated when read from.
> +
> + 56 - 63: Reserved and must be zero.
> +
> +=== Bitmap data ===
> +
> +As noted above, bitmap data is stored in separate clusters, described by the
> +bitmap table. Given an offset (in bytes) into the bitmap data, the offset
> into
> +the image file can be obtained as follows:
> +
> + image_offset =
> + bitmap_table[bitmap_data_offset / cluster_size] +
> + (bitmap_data_offset % cluster_size)
> +
> +This offset is not defined if bits 9 - 55 of bitmap table entry are zero (see
> +above).
> +
> +Given an offset byte_nr into the virtual disk and the bitmap's granularity,
> the
> +bit offset into the bitmap can be calculated like this:
> +
> + bit_offset =
> + image_offset(byte_nr / granularity / 8) * 8 +
> + (byte_nr / granularity) % 8
> +
> +If the size of the bitmap data is not a multiply of cluster size then the
> last
> +cluster of the bitmap data contains some unused tail bits. These bits must be
> +zero.
>
s/multiply/multiple/.
Looks good otherwise, pending the results of our discussion about what
to do about the extra_data_size / type-specific data fields.
Thanks!
--
—js
- [Qemu-devel] [PATCH v7] spec: add qcow2 bitmaps extension specification, Vladimir Sementsov-Ogievskiy, 2016/01/11
- Re: [Qemu-devel] [PATCH v7] spec: add qcow2 bitmaps extension specification,
John Snow <=
- Re: [Qemu-devel] [PATCH v7] spec: add qcow2 bitmaps extension specification, Eric Blake, 2016/01/14
- Re: [Qemu-devel] [PATCH v7] spec: add qcow2 bitmaps extension specification, John Snow, 2016/01/14
- Re: [Qemu-devel] [PATCH v7] spec: add qcow2 bitmaps extension specification, Vladimir Sementsov-Ogievskiy, 2016/01/16
- Re: [Qemu-devel] [PATCH v7] spec: add qcow2 bitmaps extension specification, John Snow, 2016/01/18
- Re: [Qemu-devel] [PATCH v7] spec: add qcow2 bitmaps extension specification, Eric Blake, 2016/01/18
- Re: [Qemu-devel] [PATCH v7] spec: add qcow2 bitmaps extension specification, Vladimir Sementsov-Ogievskiy, 2016/01/19
- Re: [Qemu-devel] [PATCH v7] spec: add qcow2 bitmaps extension specification, Kevin Wolf, 2016/01/19
- Re: [Qemu-devel] [PATCH v7] spec: add qcow2 bitmaps extension specification, Vladimir Sementsov-Ogievskiy, 2016/01/25