== 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 (only valid value is 2) 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. 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 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. The remaining space between the end of the header extension area and the end of the first cluster can be used for other data. Usually, the backing file name is stored there. == 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 / sizeof(uint16_t)) 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: Bit 0 - 15: Reference count of the cluster == 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 (for normal clusters): Bit 0 - 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) 62: 0 (this cluster is not compressed) 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. L2 table entry (for compressed clusters; x = 62 - (cluster_size - 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 62: 1 (this cluster is compressed using zlib) 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. If a cluster is unallocated, read requests shall read the data from the backing file. 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. variable: Unique ID string for the snapshot (not null terminated) variable: Name of the snapshot (not null terminated)