Qcow2 Image File Format
A qcow2 image file is organized in units of constant size, which are called
(host) clusters. A cluster is the unit in which all allocations are done,
both for actual guest data and for image metadata.
Likewise, the virtual disk as seen by the guest is divided into (guest) clusters of the same size.
All numbers in qcow2 are stored in Big Endian byte order.
Header
The first cluster of a qcow2 image contains the file header:
Byte 0 - 3: magic
QCOW magic string ("QFI\xfb")
4 - 7: version
Version number (valid values are 2 and 3)
8 - 15: backing_file_offset
Offset into the image file at which the backing file name
is stored (NB: The string is not null terminated). 0 if the
image doesn't have a backing file.
Note: backing files are incompatible with raw external data
files (auto-clear feature bit 1).
16 - 19: backing_file_size
Length of the backing file name in bytes. Must not be
longer than 1023 bytes. Undefined if the image doesn't have
a backing file.
20 - 23: cluster_bits
Number of bits that are used for addressing an offset
within a cluster (1 << cluster_bits is the cluster size).
Must not be less than 9 (i.e. 512 byte clusters).
Note: QEMU as of today has an implementation limit of 2 MB
as the maximum cluster size and won't be able to open images
with larger cluster sizes.
Note: if the image has Extended L2 Entries then cluster_bits
must be at least 14 (i.e. 16384 byte clusters).
24 - 31: size
Virtual disk size in bytes.
Note: QEMU has an implementation limit of 32 MB as
the maximum L1 table size. With a 2 MB cluster
size, it is unable to populate a virtual cluster
beyond 2 EB (61 bits); with a 512 byte cluster
size, it is unable to populate a virtual size
larger than 128 GB (37 bits). Meanwhile, L1/L2
table layouts limit an image to no more than 64 PB
(56 bits) of populated clusters, and an image may
hit other limits first (such as a file system's
maximum size).
32 - 35: crypt_method
0 for no encryption
1 for AES encryption
2 for LUKS encryption
36 - 39: l1_size
Number of entries in the active L1 table
40 - 47: l1_table_offset
Offset into the image file at which the active L1 table
starts. Must be aligned to a cluster boundary.
48 - 55: refcount_table_offset
Offset into the image file at which the refcount table
starts. Must be aligned to a cluster boundary.
56 - 59: refcount_table_clusters
Number of clusters that the refcount table occupies
60 - 63: nb_snapshots
Number of snapshots contained in the image
64 - 71: snapshots_offset
Offset into the image file at which the snapshot table
starts. Must be aligned to a cluster boundary.
For version 2, the header is exactly 72 bytes in length, and finishes here.
For version 3 or higher, the header length is at least 104 bytes, including
the next fields through header_length.
72 - 79: incompatible_features
Bitmask of incompatible features. An implementation must
fail to open an image if an unknown bit is set.
Bit 0: Dirty bit. If this bit is set then refcounts
may be inconsistent, make sure to scan L1/L2
tables to repair refcounts before accessing the
image.
Bit 1: Corrupt bit. If this bit is set then any data
structure may be corrupt and the image must not
be written to (unless for regaining
consistency).
Bit 2: External data file bit. If this bit is set, an
external data file is used. Guest clusters are
then stored in the external data file. For such
images, clusters in the external data file are
not refcounted. The offset field in the
Standard Cluster Descriptor must match the
guest offset and neither compressed clusters
nor internal snapshots are supported.
An External Data File Name header extension may
be present if this bit is set.
Bit 3: Compression type bit. If this bit is set,
a non-default compression is used for compressed
clusters. The compression_type field must be
present and not zero.
Bit 4: Extended L2 Entries. If this bit is set then
L2 table entries use an extended format that
allows subcluster-based allocation. See the
Extended L2 Entries section for more details.
Bits 5-63: Reserved (set to 0)
80 - 87: compatible_features
Bitmask of compatible features. An implementation can
safely ignore any unknown bits that are set.
Bit 0: Lazy refcounts bit. If this bit is set then
lazy refcount updates can be used. This means
marking the image file dirty and postponing
refcount metadata updates.
Bits 1-63: Reserved (set to 0)
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.
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.
Bit 1: Raw external data bit
If this bit is set, the external data file can
be read as a consistent standalone raw image
without looking at the qcow2 metadata.
Setting this bit has a performance impact for
some operations on the image (e.g. writing
zeros requires writing to the data file instead
of only setting the zero flag in the L2 table
entry) and conflicts with backing files.
This bit may only be set if the External Data
File bit (incompatible feature bit 1) is also
set.
Bits 2-63: Reserved (set to 0)
96 - 99: refcount_order
Describes the width of a reference count block entry (width
in bits: refcount_bits = 1 << refcount_order). For version 2
images, the order is always assumed to be 4
(i.e. refcount_bits = 16).
This value may not exceed 6 (i.e. refcount_bits = 64).
100 - 103: header_length
Length of the header structure in bytes. For version 2
images, the length is always assumed to be 72 bytes.
For version 3 it's at least 104 bytes and must be a multiple
of 8.
Additional fields (version 3 and higher)
In general, these fields are optional and may be safely ignored by the software, as well as filled by zeros (which is equal to field absence), if software needs to set field B, but does not care about field A which precedes B. More formally, additional fields have the following compatibility rules:
If the value of the additional field must not be ignored for correct handling of the file, it will be accompanied by a corresponding incompatible feature bit.
If there are no unrecognized incompatible feature bits set, an unknown additional field may be safely ignored other than preserving its value when rewriting the image header.
An explicit value of 0 will have the same behavior as when the field is not present*, if not altered by a specific incompatible bit.
(*) A field is considered not present when header_length is less than or equal
to the field’s offset. Also, all additional fields are not present for
version 2.
104: compression_type
Defines the compression method used for compressed clusters.
All compressed clusters in an image use the same compression
type.
If the incompatible bit "Compression type" is set: the field
must be present and non-zero (which means non-deflate
compression type). Otherwise, this field must not be present
or must be zero (which means deflate).
Available compression type values:
- 0: deflate <https://www.ietf.org/rfc/rfc1951.txt>
- 1: zstd <http://github.com/facebook/zstd>
The deflate compression type is called "zlib"
<https://www.zlib.net/> in QEMU. However, clusters with the
deflate compression type do not have zlib headers.
105 - 111: Padding, contents defined below.
Header padding
header_length must be a multiple of 8, which means that if the end of the last
additional field is not aligned, some padding is needed. This padding must be
zeroed, so that if some existing (or future) additional field will fall into
the padding, it will be interpreted accordingly to point [3.] of the previous
paragraph, i.e. in the same manner as when this field is not present.
Header extensions
Directly after the image header, optional sections called header extensions can be stored. Each extension has a structure like the following:
Byte 0 - 3: Header extension type:
0x00000000 - End of the header extension area
0xe2792aca - Backing file format name string
0x6803f857 - Feature name table
0x23852875 - Bitmaps extension
0x0537be77 - Full disk encryption header pointer
0x44415441 - External data file name string
other - Unknown header extension, can be safely
ignored
4 - 7: Length of the header extension data
8 - n: Header extension data
n - m: Padding to round up the header extension size to the next
multiple of 8.
Unless stated otherwise, each header extension type shall appear at most once in the same image.
If the image has a backing file then the backing file name should be stored in the remaining space between the end of the header extension area and the end of the first cluster. It is not allowed to store other data here, so that an implementation can safely modify the header and add extensions without harming data of compatible features that it doesn’t support. Compatible features that need space for additional data can use a header extension.
String header extensions
Some header extensions (such as the backing file format name and the external
data file name) are just a single string. In this case, the header extension
length is the string length and the string is not \0 terminated. (The header
extension padding can make it look like a string is \0 terminated, but
neither is padding always necessary nor is there a guarantee that zero bytes
are used for padding.)
Feature name table
The 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. Each entry looks 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 (valid
values: 0-63)
2 - 47: Feature name (padded with zeros, but not necessarily null
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 directory entries.
16 - 23: bitmap_directory_offset
Offset into the image file at which the bitmap directory
starts. Must be aligned to a cluster boundary.
Full disk encryption header pointer
The full disk encryption header must be present if, and only if, the
crypt_method header requires metadata. Currently this is only true
of the LUKS crypt method. The header extension must be absent for
other methods.
This header provides the offset at which the crypt method can store its additional data, as well as the length of such data.
Byte 0 - 7: Offset into the image file at which the encryption
header starts in bytes. Must be aligned to a cluster
boundary.
Byte 8 - 15: Length of the written encryption header in bytes.
Note actual space allocated in the qcow2 file may
be larger than this value, since it will be rounded
to the nearest multiple of the cluster size. Any
unused bytes in the allocated space will be initialized
to 0.
For the LUKS crypt method, the encryption header works as follows.
The first 592 bytes of the header clusters will contain the LUKS
partition header. This is then followed by the key material data areas.
The size of the key material data areas is determined by the number of
stripes in the key slot and key size. Refer to the LUKS format
specification (docs/on-disk-format.pdf in the cryptsetup source
package) for details of the LUKS partition header format.
In the LUKS partition header, the payload-offset field will be
calculated as normal for the LUKS spec. ie the size of the LUKS
header, plus key material regions, plus padding, relative to the
start of the LUKS header. This offset value is not required to be
qcow2 cluster aligned. Its value is currently never used in the
context of qcow2, since the qcow2 file format itself defines where
the real payload offset is, but none the less a valid payload offset
should always be present.
In the LUKS key slots header, the key-material-offset is relative
to the start of the LUKS header clusters in the qcow2 container,
not the start of the qcow2 file.
Logically the layout looks like
+-----------------------------+
| QCow2 header |
| QCow2 header extension X |
| QCow2 header extension FDE |
| QCow2 header extension ... |
| QCow2 header extension Z |
+-----------------------------+
| ....other QCow2 tables.... |
. .
. .
+-----------------------------+
| +-------------------------+ |
| | LUKS partition header | |
| +-------------------------+ |
| | LUKS key material 1 | |
| +-------------------------+ |
| | LUKS key material 2 | |
| +-------------------------+ |
| | LUKS key material ... | |
| +-------------------------+ |
| | LUKS key material 8 | |
| +-------------------------+ |
+-----------------------------+
| QCow2 cluster payload |
. .
. .
. .
| |
+-----------------------------+
Data encryption
When an encryption method is requested in the header, the image payload data must be encrypted/decrypted on every write/read. The image headers and metadata are never encrypted.
The algorithms used for encryption vary depending on the method
AES:The AES cipher, in CBC mode, with 256 bit keys.
Initialization vectors generated using plain64 method, with the virtual disk sector as the input tweak.
This format is no longer supported in QEMU system emulators, due to a number of design flaws affecting its security. It is only supported in the command line tools for the sake of back compatibility and data liberation.
LUKS:The algorithms are specified in the LUKS header.
Initialization vectors generated using the method specified in the LUKS header, with the physical disk sector as the input tweak.
Host cluster management
qcow2 manages the allocation of host clusters by maintaining a reference count for each host cluster. A refcount of 0 means that the cluster is free, 1 means that it is used, and >= 2 means that it is used and any write access must perform a COW (copy on write) operation.
The refcounts are managed in a two-level table. The first level is called refcount table and has a variable size (which is stored in the header). The refcount table can cover multiple clusters, however it needs to be contiguous in the image file.
It contains pointers to the second level structures which are called refcount blocks and are exactly one cluster in size.
Although a large enough refcount table can reserve clusters past 64 PB (56 bits) (assuming the underlying protocol can even be sized that large), note that some qcow2 metadata such as L1/L2 tables must point to clusters prior to that point.
Note
QEMU has an implementation limit of 8 MB as the maximum refcount table size. With a 2 MB cluster size and a default refcount_order of 4, it is unable to reference host resources beyond 2 EB (61 bits); in the worst case, with a 512 cluster size and refcount_order of 6, it is unable to access beyond 32 GB (35 bits).
Given an offset into the image file, the refcount of its cluster can be obtained as follows:
refcount_block_entries = (cluster_size * 8 / refcount_bits)
refcount_block_index = (offset / cluster_size) % refcount_block_entries
refcount_table_index = (offset / cluster_size) / refcount_block_entries
refcount_block = load_cluster(refcount_table[refcount_table_index]);
return refcount_block[refcount_block_index];
Refcount table entry:
Bit 0 - 8: Reserved (set to 0)
9 - 63: Bits 9-63 of the offset into the image file at which the
refcount block starts. Must be aligned to a cluster
boundary.
If this is 0, the corresponding refcount block has not yet
been allocated. All refcounts managed by this refcount block
are 0.
Refcount block entry (x = refcount_bits - 1):
Bit 0 - x: Reference count of the cluster. If refcount_bits implies a
sub-byte width, note that bit 0 means the least significant
bit in this context.
Cluster mapping
Just as for refcounts, qcow2 uses a two-level structure for the mapping of guest clusters to host clusters. They are called L1 and L2 table.
The L1 table has a variable size (stored in the header) and may use multiple clusters, however it must be contiguous in the image file. L2 tables are exactly one cluster in size.
The L1 and L2 tables have implications on the maximum virtual file size; for a given L1 table size, a larger cluster size is required for the guest to have access to more space. Furthermore, a virtual cluster must currently map to a host offset below 64 PB (56 bits) (although this limit could be relaxed by putting reserved bits into use). Additionally, as cluster size increases, the maximum host offset for a compressed cluster is reduced (a 2M cluster size requires compressed clusters to reside below 512 TB (49 bits), and this limit cannot be relaxed without an incompatible layout change).
Given an offset into the virtual disk, the offset into the image file can be obtained as follows:
l2_entries = (cluster_size / sizeof(uint64_t)) [*]
l2_index = (offset / cluster_size) % l2_entries
l1_index = (offset / cluster_size) / l2_entries
l2_table = load_cluster(l1_table[l1_index]);
cluster_offset = l2_table[l2_index];
return cluster_offset + (offset % cluster_size)
[*] this changes if Extended L2 Entries are enabled, see next section
L1 table entry:
Bit 0 - 8: Reserved (set to 0)
9 - 55: Bits 9-55 of the offset into the image file at which the L2
table starts. Must be aligned to a cluster boundary. If the
offset is 0, the L2 table and all clusters described by this
L2 table are unallocated.
56 - 62: Reserved (set to 0)
63: 0 for an L2 table that is unused or requires COW, 1 if its
refcount is exactly one. This information is only accurate
in the active L1 table.
L2 table entry:
Bit 0 - 61: Cluster descriptor
62: 0 for standard clusters
1 for compressed clusters
63: 0 for clusters that are unused, compressed or require COW.
1 for standard clusters whose refcount is exactly one.
This information is only accurate in L2 tables
that are reachable from the active L1 table.
With external data files, all guest clusters have an
implicit refcount of 1 (because of the fixed host = guest
mapping for guest cluster offsets), so this bit should be 1
for all allocated clusters.
Standard Cluster Descriptor:
Bit 0: If set to 1, the cluster reads as all zeros. The host
cluster offset can be used to describe a preallocation,
but it won't be used for reading data from this cluster,
nor is data read from the backing file if the cluster is
unallocated.
With version 2 or with extended L2 entries (see the next
section), this is always 0.
1 - 8: Reserved (set to 0)
9 - 55: Bits 9-55 of host cluster offset. Must be aligned to a
cluster boundary. If the offset is 0 and bit 63 is clear,
the cluster is unallocated. The offset may only be 0 with
bit 63 set (indicating a host cluster offset of 0) when an
external data file is used.
56 - 61: Reserved (set to 0)
Compressed Clusters Descriptor (x = 62 - (cluster_bits - 8)):
Bit 0 - x-1: Host cluster offset. This is usually _not_ aligned to a
cluster or sector boundary! If cluster_bits is
small enough that this field includes bits beyond
55, those upper bits must be set to 0.
x - 61: Number of additional 512-byte sectors used for the
compressed data, beyond the sector containing the offset
in the previous field. Some of these sectors may reside
in the next contiguous host cluster.
Note that the compressed data does not necessarily occupy
all of the bytes in the final sector; rather, decompression
stops when it has produced a cluster of data.
Another compressed cluster may map to the tail of the final
sector used by this compressed cluster.
If a cluster is unallocated, read requests shall read the data from the backing file (except if bit 0 in the Standard Cluster Descriptor is set). If there is no backing file or the backing file is smaller than the image, they shall read zeros for all parts that are not covered by the backing file.
Extended L2 Entries
An image uses Extended L2 Entries if bit 4 is set on the incompatible_features field of the header.
In these images standard data clusters are divided into 32 subclusters of the same size. They are contiguous and start from the beginning of the cluster. Subclusters can be allocated independently and the L2 entry contains information indicating the status of each one of them. Compressed data clusters don’t have subclusters so they are treated the same as in images without this feature.
The size of an extended L2 entry is 128 bits so the number of entries per table is calculated using this formula:
l2_entries = (cluster_size / (2 * sizeof(uint64_t)))
The first 64 bits have the same format as the standard L2 table entry described in the previous section, with the exception of bit 0 of the standard cluster descriptor.
The last 64 bits contain a subcluster allocation bitmap with this format:
Subcluster Allocation Bitmap (for standard clusters):
Bit 0 - 31: Allocation status (one bit per subcluster)
1: the subcluster is allocated. In this case the
host cluster offset field must contain a valid
offset.
0: the subcluster is not allocated. In this case
read requests shall go to the backing file or
return zeros if there is no backing file data.
Bits are assigned starting from the least significant
one (i.e. bit x is used for subcluster x).
32 - 63 Subcluster reads as zeros (one bit per subcluster)
1: the subcluster reads as zeros. In this case the
allocation status bit must be unset. The host
cluster offset field may or may not be set.
0: no effect.
Bits are assigned starting from the least significant
one (i.e. bit x is used for subcluster x - 32).
Subcluster Allocation Bitmap (for compressed clusters):
Bit 0 - 63: Reserved (set to 0)
Compressed clusters don't have subclusters,
so this field is not used.
Snapshots
qcow2 supports internal snapshots. Their basic principle of operation is to switch the active L1 table, so that a different set of host clusters are exposed to the guest.
When creating a snapshot, the L1 table should be copied and the refcount of all L2 tables and clusters reachable from this L1 table must be increased, so that a write causes a COW and isn’t visible in other snapshots.
When loading a snapshot, bit 63 of all entries in the new active L1 table and all L2 tables referenced by it must be reconstructed from the refcount table as it doesn’t need to be accurate in inactive L1 tables.
A directory of all snapshots is stored in the snapshot table, a contiguous area in the image file, whose starting offset and length are given by the header fields snapshots_offset and nb_snapshots. The entries of the snapshot table have variable length, depending on the length of ID, name and extra data.
Snapshot table entry:
Byte 0 - 7: Offset into the image file at which the L1 table for the
snapshot starts. Must be aligned to a cluster boundary.
8 - 11: Number of entries in the L1 table of the snapshots
12 - 13: Length of the unique ID string describing the snapshot
14 - 15: Length of the name of the snapshot
16 - 19: Time at which the snapshot was taken in seconds since the
Epoch
20 - 23: Subsecond part of the time at which the snapshot was taken
in nanoseconds
24 - 31: Time that the guest was running until the snapshot was
taken in nanoseconds
32 - 35: Size of the VM state in bytes. 0 if no VM state is saved.
If there is VM state, it starts at the first cluster
described by first L1 table entry that doesn't describe a
regular guest cluster (i.e. VM state is stored like guest
disk content, except that it is stored at offsets that are
larger than the virtual disk presented to the guest)
36 - 39: Size of extra data in the table entry (used for future
extensions of the format)
variable: Extra data for future extensions. Unknown fields must be
ignored. Currently defined are (offset relative to snapshot
table entry):
Byte 40 - 47: Size of the VM state in bytes. 0 if no VM
state is saved. If this field is present,
the 32-bit value in bytes 32-35 is ignored.
Byte 48 - 55: Virtual disk size of the snapshot in bytes
Byte 56 - 63: icount value which corresponds to
the record/replay instruction count
when the snapshot was taken. Set to -1
if icount was disabled
Version 3 images must include extra data at least up to
byte 55.
variable: Unique ID string for the snapshot (not null terminated)
variable: Name of the snapshot (not null terminated)
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.
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. Although the bitmap metadata is still
well-formed from a qcow2 perspective, the metadata
(such as the auto flag or bitmap size) or data
contents may be outdated.
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 supports only values 9 - 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 or metadata change. 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.