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