Note: this specification is copied from official documentation in github.com/qemu/qemu/docs/qcow2.txt.
- General
- Header
- Feature name table
- Bitmaps extension
- Host cluster management
- Cluster mapping
- Snapshots
- Bitmaps
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.
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.
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.
24 - 31: size
Virtual disk size in bytes
32 - 35: crypt_method
0 for no encryption
1 for AES 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.
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: 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).
Bits 2-63: 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.
Bits 1-63: 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.
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
0x6803f857 - Feature name table
0x23852875 - Bitmaps extension
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.
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 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 (valid
values: 0-63)
2 - 47: Feature name (padded with zeros, but not necessarily null
terminated if it has full length)
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.
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.
Given a 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.
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.
Given a 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)
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 a cluster that is unused or requires COW, 1 if its
refcount is exactly one. This information is only accurate
in L2 tables that are reachable from the active L1
table.
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, 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, the cluster is
unallocated.
56 - 61: Reserved (set to 0)
Compressed Clusters Descriptor (x = 62 - (cluster_bits - 8)):
Bit 0 - x: Host cluster offset. This is usually _not_ aligned to a
cluster boundary!
x+1 - 61: Compressed size of the images in sectors of 512 bytes
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.
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
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.
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.
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.
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.
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.
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.