| == 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). |
| |
| 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. |
| |
| The remaining space between the end of the header extension area and the end of |
| the first cluster can be used for the backing file name. 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 / 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 (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. |
