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spec: add qcow2 bitmaps extension specification

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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 <vsementsov@virtuozzo.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
This commit is contained in:
Vladimir Sementsov-Ogievskiy 2016-02-05 11:58:33 +03:00 committed by Kevin Wolf
parent f38738e212
commit bca5a8f462
1 changed files with 220 additions and 1 deletions
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221
docs/specs/qcow2.txt
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@ -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 must be
considered 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 +178,36 @@ 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:
Byte 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: Reserved, must be zero.
8 - 15: bitmap_directory_size
Size of the bitmap directory in bytes. It is the cumulative
size of all (nb_bitmaps) bitmap headers.
16 - 23: 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 +402,180 @@ 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 to a virtual disk. This section describes how these bitmaps are stored.
All stored bitmaps are related to the virtual disk stored in the same image, so
each bitmap size is equal to the virtual disk size.
Each bit of the bitmap is responsible for strictly defined range of the virtual
disk. For bit number bit_nr the corresponding range (in bytes) will be:
[bit_nr * bitmap_granularity .. (bit_nr + 1) * bitmap_granularity - 1]
Granularity is a property of the concrete bitmap, see below.
=== 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 lengths 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'.
2: extra_data_compatible
This flags is meaningful when the extra data is
unknown to the software (currently any extra data is
unknown to Qemu).
If it is set, the bitmap may be used as expected, extra
data must be left as is.
If it is not set, the bitmap must not be used, but
both it and its extra data be left as is.
Bits 3 - 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 should be zero. If it is non-zero the
behavior is defined by extra_data_compatible flag.
variable: extra_data
Extra data for the bitmap, occupying extra_data_size bytes.
Extra data must never contain references to clusters or in
some other way allocate additional clusters.
variable: name
The name of the bitmap (not null terminated), occupying
name_size bytes. 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. All bytes of the padding must be zero.
=== Bitmap table ===
Each bitmap is stored using a one-level structure (as opposed to two-level
structures 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 during reads.
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_data_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 image file to the corresponding bit of the bitmap can be
calculated like this:
bit_offset(byte_nr) =
image_offset(byte_nr / granularity / 8) * 8 +
(byte_nr / granularity) % 8
If the size of the bitmap data is not a multiple of the cluster size then the
last cluster of the bitmap data contains some unused tail bits. These bits must
be zero.
=== Dirty tracking bitmaps ===
Bitmaps with 'type' field equal to one are dirty tracking bitmaps.
When the virtual disk is in use dirty tracking bitmap may be 'enabled' or
'disabled'. While the bitmap is 'enabled', all writes to the virtual disk
should be reflected in the bitmap. A set bit in the bitmap means that the
corresponding range of the virtual disk (see above) was written to while the
bitmap was 'enabled'. An unset bit means that this range was not written to.
The software doesn't have to sync the bitmap in the image file with its
representation in RAM after each write. Flag 'in_use' should be set while the
bitmap is not synced.
In the image file the 'enabled' state is reflected by the 'auto' flag. If this
flag is set, the software must consider the bitmap as 'enabled' and start
tracking virtual disk changes to this bitmap from the first write to the
virtual disk. If this flag is not set then the bitmap is disabled.