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