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qemu/docs/specs/qcow2.txt
Max Reitz 7f75a07d50 docs/qcow2: Limit refcount_order to [0, 6] 
Specify the upper limit of refcount_order to be 6 (that is,
refcount_bits = 64). Any larger value does not make much sense when all
offsets, sizes, cluster counts etc. "only" have a width of 64 bit as
well, and very large values would be very difficult to support.
Therefore, just cap it at the largest reasonable value.

Suggested-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
2014-10-23 15:34:02 +02:00

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== General ==
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.
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.
Bits 0-63: 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
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.
== 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 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)
== 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.
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.
== 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.
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 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.
== 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
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.
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