On 21.12.2015 16:25, Vladimir Sementsov-Ogievskiy wrote: > The new feature for qcow2: storing bitmaps. > > Only bitmaps, relative to the virtual disk, stored in qcow2 file, should > be stored in this qcow2 file. > > Strings started from +# are RFC-strings, not to be commited of course > > Signed-off-by: Vladimir Sementsov-Ogievskiy <vsement...@virtuozzo.com> > --- > > > 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 | 160 > ++++++++++++++++++++++++++++++++++++++++++++++++++- > 1 file changed, 159 insertions(+), 1 deletion(-)
Looks good! :-)
Some comments below, but I think the general design is good now.
> diff --git a/docs/specs/qcow2.txt b/docs/specs/qcow2.txt
> index 121dfc8..3d557ee 100644
> --- a/docs/specs/qcow2.txt
> +++ b/docs/specs/qcow2.txt
> @@ -103,7 +103,19 @@ 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 is responsible for Bitmaps extension
> + consistency.
> +
> + If it is set, but there is no Bitmaps
> + extension, this should be considered as an
> + error.
> +
> + If it is not set, but there is a Bitmaps
> + extension, its data should be considered as
> + inconsistent.
> +
> + Bits 1-63: Reserved (set to 0)
>
> 96 - 99: refcount_order
> Describes the width of a reference count block entry
> (width
> @@ -123,6 +135,7 @@ 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 +179,34 @@ the header extension data. Each entry look like this:
> terminated if it has full length)
>
>
> +== Bitmaps extension ==
> +
> +Bitmaps extension is an optional header 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.
> +
> +The data of the extension should be considered as consistent only if
> +corresponding auto-clear feature bit is set (see autoclear_features above).
> +
> +The fields of Bitmaps extension are:
> +
> + 0 - 3: nb_bitmaps
> + The number of bitmaps contained in the image. Must be
> + greater 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 must be equal to
> + sum of sizes of all (nb_bitmaps) bitmap headers.
I'd rather write this as: "Size of the Bitmap Directory in bytes, i.e.
the cumulative size of all (nb_bitmaps) bitmap headers."
("It must" sounds like it's an additional restriction while it's
actually just an explanation.)
> +
> + 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 +401,120 @@ Snapshot table entry:
>
> variable: Padding to round up the snapshot table entry size to the
> next multiple of 8.
> +
> +
> +== Bitmaps ==
> +
> +The feature supports storing bitmaps in a qcow2 image. 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.
> 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.
> +
> +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:
> + 0: Dirty Tracking Bitmap
> +
> + Values 1 - 255 are reserved.
> +# Is there any reason, to make it '1 - Dirty Tracking Bitmap, 0,2-255 -
> reserved?'
Maybe something arbitrary like "But 0 feels like 'unknown' to me!". But
I can't think of anything substantial.
> +
> + 17: granularity_bits
> + Granularity bits. Valid values are: 0 - 63.
> +
> + Note: Qemu currently doesn't support granularity_bits
> + greater than 31.
> +
> + Granularity is calculated as
> + granularity = 1 << granularity_bits
> +
> + Granularity of the bitmap is how many bytes of the image
> + accounts for one bit of the bitmap.
> +
> + 18 - 19: name_size
> + Size of the bitmap name. Valid values: 1 - 1023.
> +
> + 20 - 21: extra_data_size
> + Size of type-specific extra data.
> +# 2 bytes or 4 bytes ?
I'd make it 4. I don't think those additional two bytes will hurt
anybody, and the simple fact that you're asking that question shows to
me that there is a possibility that 2 won't be enough. ;-)
> +
> + variable: The name of the bitmap (not null terminated). Must be
> + unique among all bitmap names within the Bitmaps
> extension.
> +
> + variable: Type-specific extra data for the bitmap.
I'd swap name and type-specific data so we know the alignment of the
type-specific data (if they are swapped and sizeof(extra_data_size) ==
4, its alignment will be 4). Maybe that'll come in handy at some point.
Maybe you should make a note that currently, no extra data is defined
for Dirty Tracking Bitmaps, but that this may change in the future.
Max
> + 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 (not two-level like refcounts and guest
> +clusters mapping) structure for the mapping of bitmaps data to host clusters.
> +It is called 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.
> +
> +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 host cluster offset. Must be aligned to a
> + cluster boundary. If the offset is 0, the cluster is
> + unallocated, see bit 0 description.
> +
> + 56 - 63: Reserved and must be zero.
> +
> +=== Bitmap Data ===
> +
> +As noted above, bitmap data is stored in several (or may be one, exactly
> +bitmap_table_size) separate clusters, described by 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)
> +
> +Taking into account the granularity of the bitmap, an offset in bits into the
> +image file, corresponding to byte number byte_nr of the virtual disk can be
> +calculated like this:
> +
> + bit_offset =
> + image_offset(byte_nr / granularity / 8) * 8 +
> + (byte_nr / granularity) % 8
>
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