--- /dev/null
+== 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.
Code Review / kvmfornfv.git / blobdiff