docs/specs/qcow2.txt

   1 == General ==
   2
   3 A qcow2 image file is organized in units of constant size, which are called
   4 (host) clusters. A cluster is the unit in which all allocations are done,
   5 both for actual guest data and for image metadata.
   6
   7 Likewise, the virtual disk as seen by the guest is divided into (guest)
   8 clusters of the same size.
   9
  10 All numbers in qcow2 are stored in Big Endian byte order.
  11
  12
  13 == Header ==
  14
  15 The first cluster of a qcow2 image contains the file header:
  16
  17     Byte  0 -  3:   magic
  18                     QCOW magic string ("QFI\xfb")
  19
  20           4 -  7:   version
  21                     Version number (valid values are 2 and 3)
  22
  23           8 - 15:   backing_file_offset
  24                     Offset into the image file at which the backing file name
  25                     is stored (NB: The string is not null terminated). 0 if the
  26                     image doesn't have a backing file.
  27
  28          16 - 19:   backing_file_size
  29                     Length of the backing file name in bytes. Must not be
  30                     longer than 1023 bytes. Undefined if the image doesn't have
  31                     a backing file.
  32
  33          20 - 23:   cluster_bits
  34                     Number of bits that are used for addressing an offset
  35                     within a cluster (1 << cluster_bits is the cluster size).
  36                     Must not be less than 9 (i.e. 512 byte clusters).
  37
  38                     Note: qemu as of today has an implementation limit of 2 MB
  39                     as the maximum cluster size and won't be able to open images
  40                     with larger cluster sizes.
  41
  42          24 - 31:   size
  43                     Virtual disk size in bytes
  44
  45          32 - 35:   crypt_method
  46                     0 for no encryption
  47                     1 for AES encryption
  48
  49          36 - 39:   l1_size
  50                     Number of entries in the active L1 table
  51
  52          40 - 47:   l1_table_offset
  53                     Offset into the image file at which the active L1 table
  54                     starts. Must be aligned to a cluster boundary.
  55
  56          48 - 55:   refcount_table_offset
  57                     Offset into the image file at which the refcount table
  58                     starts. Must be aligned to a cluster boundary.
  59
  60          56 - 59:   refcount_table_clusters
  61                     Number of clusters that the refcount table occupies
  62
  63          60 - 63:   nb_snapshots
  64                     Number of snapshots contained in the image
  65
  66          64 - 71:   snapshots_offset
  67                     Offset into the image file at which the snapshot table
  68                     starts. Must be aligned to a cluster boundary.
  69
  70 If the version is 3 or higher, the header has the following additional fields.
  71 For version 2, the values are assumed to be zero, unless specified otherwise
  72 in the description of a field.
  73
  74          72 -  79:  incompatible_features
  75                     Bitmask of incompatible features. An implementation must
  76                     fail to open an image if an unknown bit is set.
  77
  78                     Bit 0:      Dirty bit.  If this bit is set then refcounts
  79                                 may be inconsistent, make sure to scan L1/L2
  80                                 tables to repair refcounts before accessing the
  81                                 image.
  82
  83                     Bit 1:      Corrupt bit.  If this bit is set then any data
  84                                 structure may be corrupt and the image must not
  85                                 be written to (unless for regaining
  86                                 consistency).
  87
  88                     Bits 2-63:  Reserved (set to 0)
  89
  90          80 -  87:  compatible_features
  91                     Bitmask of compatible features. An implementation can
  92                     safely ignore any unknown bits that are set.
  93
  94                     Bit 0:      Lazy refcounts bit.  If this bit is set then
  95                                 lazy refcount updates can be used.  This means
  96                                 marking the image file dirty and postponing
  97                                 refcount metadata updates.
  98
  99                     Bits 1-63:  Reserved (set to 0)
 100
 101          88 -  95:  autoclear_features
 102                     Bitmask of auto-clear features. An implementation may only
 103                     write to an image with unknown auto-clear features if it
 104                     clears the respective bits from this field first.
 105
 106                     Bits 0-63:  Reserved (set to 0)
 107
 108          96 -  99:  refcount_order
 109                     Describes the width of a reference count block entry (width
 110                     in bits = 1 << refcount_order). For version 2 images, the
 111                     order is always assumed to be 4 (i.e. the width is 16 bits).
 112
 113         100 - 103:  header_length
 114                     Length of the header structure in bytes. For version 2
 115                     images, the length is always assumed to be 72 bytes.
 116
 117 Directly after the image header, optional sections called header extensions can
 118 be stored. Each extension has a structure like the following:
 119
 120     Byte  0 -  3:   Header extension type:
 121                         0x00000000 - End of the header extension area
 122                         0xE2792ACA - Backing file format name
 123                         0x6803f857 - Feature name table
 124                         other      - Unknown header extension, can be safely
 125                                      ignored
 126
 127           4 -  7:   Length of the header extension data
 128
 129           8 -  n:   Header extension data
 130
 131           n -  m:   Padding to round up the header extension size to the next
 132                     multiple of 8.
 133
 134 Unless stated otherwise, each header extension type shall appear at most once
 135 in the same image.
 136
 137 The remaining space between the end of the header extension area and the end of
 138 the first cluster can be used for the backing file name. It is not allowed to
 139 store other data here, so that an implementation can safely modify the header
 140 and add extensions without harming data of compatible features that it
 141 doesn't support. Compatible features that need space for additional data can
 142 use a header extension.
 143
 144
 145 == Feature name table ==
 146
 147 The feature name table is an optional header extension that contains the name
 148 for features used by the image. It can be used by applications that don't know
 149 the respective feature (e.g. because the feature was introduced only later) to
 150 display a useful error message.
 151
 152 The number of entries in the feature name table is determined by the length of
 153 the header extension data. Each entry look like this:
 154
 155     Byte       0:   Type of feature (select feature bitmap)
 156                         0: Incompatible feature
 157                         1: Compatible feature
 158                         2: Autoclear feature
 159
 160                1:   Bit number within the selected feature bitmap (valid
 161                     values: 0-63)
 162
 163           2 - 47:   Feature name (padded with zeros, but not necessarily null
 164                     terminated if it has full length)
 165
 166
 167 == Host cluster management ==
 168
 169 qcow2 manages the allocation of host clusters by maintaining a reference count
 170 for each host cluster. A refcount of 0 means that the cluster is free, 1 means
 171 that it is used, and >= 2 means that it is used and any write access must
 172 perform a COW (copy on write) operation.
 173
 174 The refcounts are managed in a two-level table. The first level is called
 175 refcount table and has a variable size (which is stored in the header). The
 176 refcount table can cover multiple clusters, however it needs to be contiguous
 177 in the image file.
 178
 179 It contains pointers to the second level structures which are called refcount
 180 blocks and are exactly one cluster in size.
 181
 182 Given a offset into the image file, the refcount of its cluster can be obtained
 183 as follows:
 184
 185     refcount_block_entries = (cluster_size / sizeof(uint16_t))
 186
 187     refcount_block_index = (offset / cluster_size) % refcount_block_entries
 188     refcount_table_index = (offset / cluster_size) / refcount_block_entries
 189
 190     refcount_block = load_cluster(refcount_table[refcount_table_index]);
 191     return refcount_block[refcount_block_index];
 192
 193 Refcount table entry:
 194
 195     Bit  0 -  8:    Reserved (set to 0)
 196
 197          9 - 63:    Bits 9-63 of the offset into the image file at which the
 198                     refcount block starts. Must be aligned to a cluster
 199                     boundary.
 200
 201                     If this is 0, the corresponding refcount block has not yet
 202                     been allocated. All refcounts managed by this refcount block
 203                     are 0.
 204
 205 Refcount block entry (x = refcount_bits - 1):
 206
 207     Bit  0 -  x:    Reference count of the cluster. If refcount_bits implies a
 208                     sub-byte width, note that bit 0 means the least significant
 209                     bit in this context.
 210
 211
 212 == Cluster mapping ==
 213
 214 Just as for refcounts, qcow2 uses a two-level structure for the mapping of
 215 guest clusters to host clusters. They are called L1 and L2 table.
 216
 217 The L1 table has a variable size (stored in the header) and may use multiple
 218 clusters, however it must be contiguous in the image file. L2 tables are
 219 exactly one cluster in size.
 220
 221 Given a offset into the virtual disk, the offset into the image file can be
 222 obtained as follows:
 223
 224     l2_entries = (cluster_size / sizeof(uint64_t))
 225
 226     l2_index = (offset / cluster_size) % l2_entries
 227     l1_index = (offset / cluster_size) / l2_entries
 228
 229     l2_table = load_cluster(l1_table[l1_index]);
 230     cluster_offset = l2_table[l2_index];
 231
 232     return cluster_offset + (offset % cluster_size)
 233
 234 L1 table entry:
 235
 236     Bit  0 -  8:    Reserved (set to 0)
 237
 238          9 - 55:    Bits 9-55 of the offset into the image file at which the L2
 239                     table starts. Must be aligned to a cluster boundary. If the
 240                     offset is 0, the L2 table and all clusters described by this
 241                     L2 table are unallocated.
 242
 243         56 - 62:    Reserved (set to 0)
 244
 245              63:    0 for an L2 table that is unused or requires COW, 1 if its
 246                     refcount is exactly one. This information is only accurate
 247                     in the active L1 table.
 248
 249 L2 table entry:
 250
 251     Bit  0 -  61:   Cluster descriptor
 252
 253               62:   0 for standard clusters
 254                     1 for compressed clusters
 255
 256               63:   0 for a cluster that is unused or requires COW, 1 if its
 257                     refcount is exactly one. This information is only accurate
 258                     in L2 tables that are reachable from the the active L1
 259                     table.
 260
 261 Standard Cluster Descriptor:
 262
 263     Bit       0:    If set to 1, the cluster reads as all zeros. The host
 264                     cluster offset can be used to describe a preallocation,
 265                     but it won't be used for reading data from this cluster,
 266                     nor is data read from the backing file if the cluster is
 267                     unallocated.
 268
 269                     With version 2, this is always 0.
 270
 271          1 -  8:    Reserved (set to 0)
 272
 273          9 - 55:    Bits 9-55 of host cluster offset. Must be aligned to a
 274                     cluster boundary. If the offset is 0, the cluster is
 275                     unallocated.
 276
 277         56 - 61:    Reserved (set to 0)
 278
 279
 280 Compressed Clusters Descriptor (x = 62 - (cluster_bits - 8)):
 281
 282     Bit  0 -  x:    Host cluster offset. This is usually _not_ aligned to a
 283                     cluster boundary!
 284
 285        x+1 - 61:    Compressed size of the images in sectors of 512 bytes
 286
 287 If a cluster is unallocated, read requests shall read the data from the backing
 288 file (except if bit 0 in the Standard Cluster Descriptor is set). If there is
 289 no backing file or the backing file is smaller than the image, they shall read
 290 zeros for all parts that are not covered by the backing file.
 291
 292
 293 == Snapshots ==
 294
 295 qcow2 supports internal snapshots. Their basic principle of operation is to
 296 switch the active L1 table, so that a different set of host clusters are
 297 exposed to the guest.
 298
 299 When creating a snapshot, the L1 table should be copied and the refcount of all
 300 L2 tables and clusters reachable from this L1 table must be increased, so that
 301 a write causes a COW and isn't visible in other snapshots.
 302
 303 When loading a snapshot, bit 63 of all entries in the new active L1 table and
 304 all L2 tables referenced by it must be reconstructed from the refcount table
 305 as it doesn't need to be accurate in inactive L1 tables.
 306
 307 A directory of all snapshots is stored in the snapshot table, a contiguous area
 308 in the image file, whose starting offset and length are given by the header
 309 fields snapshots_offset and nb_snapshots. The entries of the snapshot table
 310 have variable length, depending on the length of ID, name and extra data.
 311
 312 Snapshot table entry:
 313
 314     Byte 0 -  7:    Offset into the image file at which the L1 table for the
 315                     snapshot starts. Must be aligned to a cluster boundary.
 316
 317          8 - 11:    Number of entries in the L1 table of the snapshots
 318
 319         12 - 13:    Length of the unique ID string describing the snapshot
 320
 321         14 - 15:    Length of the name of the snapshot
 322
 323         16 - 19:    Time at which the snapshot was taken in seconds since the
 324                     Epoch
 325
 326         20 - 23:    Subsecond part of the time at which the snapshot was taken
 327                     in nanoseconds
 328
 329         24 - 31:    Time that the guest was running until the snapshot was
 330                     taken in nanoseconds
 331
 332         32 - 35:    Size of the VM state in bytes. 0 if no VM state is saved.
 333                     If there is VM state, it starts at the first cluster
 334                     described by first L1 table entry that doesn't describe a
 335                     regular guest cluster (i.e. VM state is stored like guest
 336                     disk content, except that it is stored at offsets that are
 337                     larger than the virtual disk presented to the guest)
 338
 339         36 - 39:    Size of extra data in the table entry (used for future
 340                     extensions of the format)
 341
 342         variable:   Extra data for future extensions. Unknown fields must be
 343                     ignored. Currently defined are (offset relative to snapshot
 344                     table entry):
 345
 346                     Byte 40 - 47:   Size of the VM state in bytes. 0 if no VM
 347                                     state is saved. If this field is present,
 348                                     the 32-bit value in bytes 32-35 is ignored.
 349
 350                     Byte 48 - 55:   Virtual disk size of the snapshot in bytes
 351
 352                     Version 3 images must include extra data at least up to
 353                     byte 55.
 354
 355         variable:   Unique ID string for the snapshot (not null terminated)
 356
 357         variable:   Name of the snapshot (not null terminated)
 358
 359         variable:   Padding to round up the snapshot table entry size to the
 360                     next multiple of 8.