1== General == 2 3A 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, 5both for actual guest data and for image metadata. 6 7Likewise, the virtual disk as seen by the guest is divided into (guest) 8clusters of the same size. 9 10All numbers in qcow2 are stored in Big Endian byte order. 11 12 13== Header == 14 15The 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 2 for LUKS encryption 49 50 36 - 39: l1_size 51 Number of entries in the active L1 table 52 53 40 - 47: l1_table_offset 54 Offset into the image file at which the active L1 table 55 starts. Must be aligned to a cluster boundary. 56 57 48 - 55: refcount_table_offset 58 Offset into the image file at which the refcount table 59 starts. Must be aligned to a cluster boundary. 60 61 56 - 59: refcount_table_clusters 62 Number of clusters that the refcount table occupies 63 64 60 - 63: nb_snapshots 65 Number of snapshots contained in the image 66 67 64 - 71: snapshots_offset 68 Offset into the image file at which the snapshot table 69 starts. Must be aligned to a cluster boundary. 70 71If the version is 3 or higher, the header has the following additional fields. 72For version 2, the values are assumed to be zero, unless specified otherwise 73in the description of a field. 74 75 72 - 79: incompatible_features 76 Bitmask of incompatible features. An implementation must 77 fail to open an image if an unknown bit is set. 78 79 Bit 0: Dirty bit. If this bit is set then refcounts 80 may be inconsistent, make sure to scan L1/L2 81 tables to repair refcounts before accessing the 82 image. 83 84 Bit 1: Corrupt bit. If this bit is set then any data 85 structure may be corrupt and the image must not 86 be written to (unless for regaining 87 consistency). 88 89 Bits 2-63: Reserved (set to 0) 90 91 80 - 87: compatible_features 92 Bitmask of compatible features. An implementation can 93 safely ignore any unknown bits that are set. 94 95 Bit 0: Lazy refcounts bit. If this bit is set then 96 lazy refcount updates can be used. This means 97 marking the image file dirty and postponing 98 refcount metadata updates. 99 100 Bits 1-63: Reserved (set to 0) 101 102 88 - 95: autoclear_features 103 Bitmask of auto-clear features. An implementation may only 104 write to an image with unknown auto-clear features if it 105 clears the respective bits from this field first. 106 107 Bit 0: Bitmaps extension bit 108 This bit indicates consistency for the bitmaps 109 extension data. 110 111 It is an error if this bit is set without the 112 bitmaps extension present. 113 114 If the bitmaps extension is present but this 115 bit is unset, the bitmaps extension data must be 116 considered inconsistent. 117 118 Bits 1-63: Reserved (set to 0) 119 120 96 - 99: refcount_order 121 Describes the width of a reference count block entry (width 122 in bits: refcount_bits = 1 << refcount_order). For version 2 123 images, the order is always assumed to be 4 124 (i.e. refcount_bits = 16). 125 This value may not exceed 6 (i.e. refcount_bits = 64). 126 127 100 - 103: header_length 128 Length of the header structure in bytes. For version 2 129 images, the length is always assumed to be 72 bytes. 130 131Directly after the image header, optional sections called header extensions can 132be stored. Each extension has a structure like the following: 133 134 Byte 0 - 3: Header extension type: 135 0x00000000 - End of the header extension area 136 0xE2792ACA - Backing file format name 137 0x6803f857 - Feature name table 138 0x23852875 - Bitmaps extension 139 0x0537be77 - Full disk encryption header pointer 140 other - Unknown header extension, can be safely 141 ignored 142 143 4 - 7: Length of the header extension data 144 145 8 - n: Header extension data 146 147 n - m: Padding to round up the header extension size to the next 148 multiple of 8. 149 150Unless stated otherwise, each header extension type shall appear at most once 151in the same image. 152 153If the image has a backing file then the backing file name should be stored in 154the remaining space between the end of the header extension area and the end of 155the first cluster. It is not allowed to store other data here, so that an 156implementation can safely modify the header and add extensions without harming 157data of compatible features that it doesn't support. Compatible features that 158need space for additional data can use a header extension. 159 160 161== Feature name table == 162 163The feature name table is an optional header extension that contains the name 164for features used by the image. It can be used by applications that don't know 165the respective feature (e.g. because the feature was introduced only later) to 166display a useful error message. 167 168The number of entries in the feature name table is determined by the length of 169the header extension data. Each entry look like this: 170 171 Byte 0: Type of feature (select feature bitmap) 172 0: Incompatible feature 173 1: Compatible feature 174 2: Autoclear feature 175 176 1: Bit number within the selected feature bitmap (valid 177 values: 0-63) 178 179 2 - 47: Feature name (padded with zeros, but not necessarily null 180 terminated if it has full length) 181 182 183== Bitmaps extension == 184 185The bitmaps extension is an optional header extension. It provides the ability 186to store bitmaps related to a virtual disk. For now, there is only one bitmap 187type: the dirty tracking bitmap, which tracks virtual disk changes from some 188point in time. 189 190The data of the extension should be considered consistent only if the 191corresponding auto-clear feature bit is set, see autoclear_features above. 192 193The fields of the bitmaps extension are: 194 195 Byte 0 - 3: nb_bitmaps 196 The number of bitmaps contained in the image. Must be 197 greater than or equal to 1. 198 199 Note: Qemu currently only supports up to 65535 bitmaps per 200 image. 201 202 4 - 7: Reserved, must be zero. 203 204 8 - 15: bitmap_directory_size 205 Size of the bitmap directory in bytes. It is the cumulative 206 size of all (nb_bitmaps) bitmap directory entries. 207 208 16 - 23: bitmap_directory_offset 209 Offset into the image file at which the bitmap directory 210 starts. Must be aligned to a cluster boundary. 211 212== Full disk encryption header pointer == 213 214The full disk encryption header must be present if, and only if, the 215'crypt_method' header requires metadata. Currently this is only true 216of the 'LUKS' crypt method. The header extension must be absent for 217other methods. 218 219This header provides the offset at which the crypt method can store 220its additional data, as well as the length of such data. 221 222 Byte 0 - 7: Offset into the image file at which the encryption 223 header starts in bytes. Must be aligned to a cluster 224 boundary. 225 Byte 8 - 15: Length of the written encryption header in bytes. 226 Note actual space allocated in the qcow2 file may 227 be larger than this value, since it will be rounded 228 to the nearest multiple of the cluster size. Any 229 unused bytes in the allocated space will be initialized 230 to 0. 231 232For the LUKS crypt method, the encryption header works as follows. 233 234The first 592 bytes of the header clusters will contain the LUKS 235partition header. This is then followed by the key material data areas. 236The size of the key material data areas is determined by the number of 237stripes in the key slot and key size. Refer to the LUKS format 238specification ('docs/on-disk-format.pdf' in the cryptsetup source 239package) for details of the LUKS partition header format. 240 241In the LUKS partition header, the "payload-offset" field will be 242calculated as normal for the LUKS spec. ie the size of the LUKS 243header, plus key material regions, plus padding, relative to the 244start of the LUKS header. This offset value is not required to be 245qcow2 cluster aligned. Its value is currently never used in the 246context of qcow2, since the qcow2 file format itself defines where 247the real payload offset is, but none the less a valid payload offset 248should always be present. 249 250In the LUKS key slots header, the "key-material-offset" is relative 251to the start of the LUKS header clusters in the qcow2 container, 252not the start of the qcow2 file. 253 254Logically the layout looks like 255 256 +-----------------------------+ 257 | QCow2 header | 258 | QCow2 header extension X | 259 | QCow2 header extension FDE | 260 | QCow2 header extension ... | 261 | QCow2 header extension Z | 262 +-----------------------------+ 263 | ....other QCow2 tables.... | 264 . . 265 . . 266 +-----------------------------+ 267 | +-------------------------+ | 268 | | LUKS partition header | | 269 | +-------------------------+ | 270 | | LUKS key material 1 | | 271 | +-------------------------+ | 272 | | LUKS key material 2 | | 273 | +-------------------------+ | 274 | | LUKS key material ... | | 275 | +-------------------------+ | 276 | | LUKS key material 8 | | 277 | +-------------------------+ | 278 +-----------------------------+ 279 | QCow2 cluster payload | 280 . . 281 . . 282 . . 283 | | 284 +-----------------------------+ 285 286== Data encryption == 287 288When an encryption method is requested in the header, the image payload 289data must be encrypted/decrypted on every write/read. The image headers 290and metadata are never encrypted. 291 292The algorithms used for encryption vary depending on the method 293 294 - AES: 295 296 The AES cipher, in CBC mode, with 256 bit keys. 297 298 Initialization vectors generated using plain64 method, with 299 the virtual disk sector as the input tweak. 300 301 This format is no longer supported in QEMU system emulators, due 302 to a number of design flaws affecting its security. It is only 303 supported in the command line tools for the sake of back compatibility 304 and data liberation. 305 306 - LUKS: 307 308 The algorithms are specified in the LUKS header. 309 310 Initialization vectors generated using the method specified 311 in the LUKS header, with the physical disk sector as the 312 input tweak. 313 314== Host cluster management == 315 316qcow2 manages the allocation of host clusters by maintaining a reference count 317for each host cluster. A refcount of 0 means that the cluster is free, 1 means 318that it is used, and >= 2 means that it is used and any write access must 319perform a COW (copy on write) operation. 320 321The refcounts are managed in a two-level table. The first level is called 322refcount table and has a variable size (which is stored in the header). The 323refcount table can cover multiple clusters, however it needs to be contiguous 324in the image file. 325 326It contains pointers to the second level structures which are called refcount 327blocks and are exactly one cluster in size. 328 329Given a offset into the image file, the refcount of its cluster can be obtained 330as follows: 331 332 refcount_block_entries = (cluster_size * 8 / refcount_bits) 333 334 refcount_block_index = (offset / cluster_size) % refcount_block_entries 335 refcount_table_index = (offset / cluster_size) / refcount_block_entries 336 337 refcount_block = load_cluster(refcount_table[refcount_table_index]); 338 return refcount_block[refcount_block_index]; 339 340Refcount table entry: 341 342 Bit 0 - 8: Reserved (set to 0) 343 344 9 - 63: Bits 9-63 of the offset into the image file at which the 345 refcount block starts. Must be aligned to a cluster 346 boundary. 347 348 If this is 0, the corresponding refcount block has not yet 349 been allocated. All refcounts managed by this refcount block 350 are 0. 351 352Refcount block entry (x = refcount_bits - 1): 353 354 Bit 0 - x: Reference count of the cluster. If refcount_bits implies a 355 sub-byte width, note that bit 0 means the least significant 356 bit in this context. 357 358 359== Cluster mapping == 360 361Just as for refcounts, qcow2 uses a two-level structure for the mapping of 362guest clusters to host clusters. They are called L1 and L2 table. 363 364The L1 table has a variable size (stored in the header) and may use multiple 365clusters, however it must be contiguous in the image file. L2 tables are 366exactly one cluster in size. 367 368Given a offset into the virtual disk, the offset into the image file can be 369obtained as follows: 370 371 l2_entries = (cluster_size / sizeof(uint64_t)) 372 373 l2_index = (offset / cluster_size) % l2_entries 374 l1_index = (offset / cluster_size) / l2_entries 375 376 l2_table = load_cluster(l1_table[l1_index]); 377 cluster_offset = l2_table[l2_index]; 378 379 return cluster_offset + (offset % cluster_size) 380 381L1 table entry: 382 383 Bit 0 - 8: Reserved (set to 0) 384 385 9 - 55: Bits 9-55 of the offset into the image file at which the L2 386 table starts. Must be aligned to a cluster boundary. If the 387 offset is 0, the L2 table and all clusters described by this 388 L2 table are unallocated. 389 390 56 - 62: Reserved (set to 0) 391 392 63: 0 for an L2 table that is unused or requires COW, 1 if its 393 refcount is exactly one. This information is only accurate 394 in the active L1 table. 395 396L2 table entry: 397 398 Bit 0 - 61: Cluster descriptor 399 400 62: 0 for standard clusters 401 1 for compressed clusters 402 403 63: 0 for a cluster that is unused or requires COW, 1 if its 404 refcount is exactly one. This information is only accurate 405 in L2 tables that are reachable from the active L1 406 table. 407 408Standard Cluster Descriptor: 409 410 Bit 0: If set to 1, the cluster reads as all zeros. The host 411 cluster offset can be used to describe a preallocation, 412 but it won't be used for reading data from this cluster, 413 nor is data read from the backing file if the cluster is 414 unallocated. 415 416 With version 2, this is always 0. 417 418 1 - 8: Reserved (set to 0) 419 420 9 - 55: Bits 9-55 of host cluster offset. Must be aligned to a 421 cluster boundary. If the offset is 0, the cluster is 422 unallocated. 423 424 56 - 61: Reserved (set to 0) 425 426 427Compressed Clusters Descriptor (x = 62 - (cluster_bits - 8)): 428 429 Bit 0 - x: Host cluster offset. This is usually _not_ aligned to a 430 cluster boundary! 431 432 x+1 - 61: Compressed size of the images in sectors of 512 bytes 433 434If a cluster is unallocated, read requests shall read the data from the backing 435file (except if bit 0 in the Standard Cluster Descriptor is set). If there is 436no backing file or the backing file is smaller than the image, they shall read 437zeros for all parts that are not covered by the backing file. 438 439 440== Snapshots == 441 442qcow2 supports internal snapshots. Their basic principle of operation is to 443switch the active L1 table, so that a different set of host clusters are 444exposed to the guest. 445 446When creating a snapshot, the L1 table should be copied and the refcount of all 447L2 tables and clusters reachable from this L1 table must be increased, so that 448a write causes a COW and isn't visible in other snapshots. 449 450When loading a snapshot, bit 63 of all entries in the new active L1 table and 451all L2 tables referenced by it must be reconstructed from the refcount table 452as it doesn't need to be accurate in inactive L1 tables. 453 454A directory of all snapshots is stored in the snapshot table, a contiguous area 455in the image file, whose starting offset and length are given by the header 456fields snapshots_offset and nb_snapshots. The entries of the snapshot table 457have variable length, depending on the length of ID, name and extra data. 458 459Snapshot table entry: 460 461 Byte 0 - 7: Offset into the image file at which the L1 table for the 462 snapshot starts. Must be aligned to a cluster boundary. 463 464 8 - 11: Number of entries in the L1 table of the snapshots 465 466 12 - 13: Length of the unique ID string describing the snapshot 467 468 14 - 15: Length of the name of the snapshot 469 470 16 - 19: Time at which the snapshot was taken in seconds since the 471 Epoch 472 473 20 - 23: Subsecond part of the time at which the snapshot was taken 474 in nanoseconds 475 476 24 - 31: Time that the guest was running until the snapshot was 477 taken in nanoseconds 478 479 32 - 35: Size of the VM state in bytes. 0 if no VM state is saved. 480 If there is VM state, it starts at the first cluster 481 described by first L1 table entry that doesn't describe a 482 regular guest cluster (i.e. VM state is stored like guest 483 disk content, except that it is stored at offsets that are 484 larger than the virtual disk presented to the guest) 485 486 36 - 39: Size of extra data in the table entry (used for future 487 extensions of the format) 488 489 variable: Extra data for future extensions. Unknown fields must be 490 ignored. Currently defined are (offset relative to snapshot 491 table entry): 492 493 Byte 40 - 47: Size of the VM state in bytes. 0 if no VM 494 state is saved. If this field is present, 495 the 32-bit value in bytes 32-35 is ignored. 496 497 Byte 48 - 55: Virtual disk size of the snapshot in bytes 498 499 Version 3 images must include extra data at least up to 500 byte 55. 501 502 variable: Unique ID string for the snapshot (not null terminated) 503 504 variable: Name of the snapshot (not null terminated) 505 506 variable: Padding to round up the snapshot table entry size to the 507 next multiple of 8. 508 509 510== Bitmaps == 511 512As mentioned above, the bitmaps extension provides the ability to store bitmaps 513related to a virtual disk. This section describes how these bitmaps are stored. 514 515All stored bitmaps are related to the virtual disk stored in the same image, so 516each bitmap size is equal to the virtual disk size. 517 518Each bit of the bitmap is responsible for strictly defined range of the virtual 519disk. For bit number bit_nr the corresponding range (in bytes) will be: 520 521 [bit_nr * bitmap_granularity .. (bit_nr + 1) * bitmap_granularity - 1] 522 523Granularity is a property of the concrete bitmap, see below. 524 525 526=== Bitmap directory === 527 528Each bitmap saved in the image is described in a bitmap directory entry. The 529bitmap directory is a contiguous area in the image file, whose starting offset 530and length are given by the header extension fields bitmap_directory_offset and 531bitmap_directory_size. The entries of the bitmap directory have variable 532length, depending on the lengths of the bitmap name and extra data. 533 534Structure of a bitmap directory entry: 535 536 Byte 0 - 7: bitmap_table_offset 537 Offset into the image file at which the bitmap table 538 (described below) for the bitmap starts. Must be aligned to 539 a cluster boundary. 540 541 8 - 11: bitmap_table_size 542 Number of entries in the bitmap table of the bitmap. 543 544 12 - 15: flags 545 Bit 546 0: in_use 547 The bitmap was not saved correctly and may be 548 inconsistent. 549 550 1: auto 551 The bitmap must reflect all changes of the virtual 552 disk by any application that would write to this qcow2 553 file (including writes, snapshot switching, etc.). The 554 type of this bitmap must be 'dirty tracking bitmap'. 555 556 2: extra_data_compatible 557 This flags is meaningful when the extra data is 558 unknown to the software (currently any extra data is 559 unknown to Qemu). 560 If it is set, the bitmap may be used as expected, extra 561 data must be left as is. 562 If it is not set, the bitmap must not be used, but 563 both it and its extra data be left as is. 564 565 Bits 3 - 31 are reserved and must be 0. 566 567 16: type 568 This field describes the sort of the bitmap. 569 Values: 570 1: Dirty tracking bitmap 571 572 Values 0, 2 - 255 are reserved. 573 574 17: granularity_bits 575 Granularity bits. Valid values: 0 - 63. 576 577 Note: Qemu currently supports only values 9 - 31. 578 579 Granularity is calculated as 580 granularity = 1 << granularity_bits 581 582 A bitmap's granularity is how many bytes of the image 583 accounts for one bit of the bitmap. 584 585 18 - 19: name_size 586 Size of the bitmap name. Must be non-zero. 587 588 Note: Qemu currently doesn't support values greater than 589 1023. 590 591 20 - 23: extra_data_size 592 Size of type-specific extra data. 593 594 For now, as no extra data is defined, extra_data_size is 595 reserved and should be zero. If it is non-zero the 596 behavior is defined by extra_data_compatible flag. 597 598 variable: extra_data 599 Extra data for the bitmap, occupying extra_data_size bytes. 600 Extra data must never contain references to clusters or in 601 some other way allocate additional clusters. 602 603 variable: name 604 The name of the bitmap (not null terminated), occupying 605 name_size bytes. Must be unique among all bitmap names 606 within the bitmaps extension. 607 608 variable: Padding to round up the bitmap directory entry size to the 609 next multiple of 8. All bytes of the padding must be zero. 610 611 612=== Bitmap table === 613 614Each bitmap is stored using a one-level structure (as opposed to two-level 615structures like for refcounts and guest clusters mapping) for the mapping of 616bitmap data to host clusters. This structure is called the bitmap table. 617 618Each bitmap table has a variable size (stored in the bitmap directory entry) 619and may use multiple clusters, however, it must be contiguous in the image 620file. 621 622Structure of a bitmap table entry: 623 624 Bit 0: Reserved and must be zero if bits 9 - 55 are non-zero. 625 If bits 9 - 55 are zero: 626 0: Cluster should be read as all zeros. 627 1: Cluster should be read as all ones. 628 629 1 - 8: Reserved and must be zero. 630 631 9 - 55: Bits 9 - 55 of the host cluster offset. Must be aligned to 632 a cluster boundary. If the offset is 0, the cluster is 633 unallocated; in that case, bit 0 determines how this 634 cluster should be treated during reads. 635 636 56 - 63: Reserved and must be zero. 637 638 639=== Bitmap data === 640 641As noted above, bitmap data is stored in separate clusters, described by the 642bitmap table. Given an offset (in bytes) into the bitmap data, the offset into 643the image file can be obtained as follows: 644 645 image_offset(bitmap_data_offset) = 646 bitmap_table[bitmap_data_offset / cluster_size] + 647 (bitmap_data_offset % cluster_size) 648 649This offset is not defined if bits 9 - 55 of bitmap table entry are zero (see 650above). 651 652Given an offset byte_nr into the virtual disk and the bitmap's granularity, the 653bit offset into the image file to the corresponding bit of the bitmap can be 654calculated like this: 655 656 bit_offset(byte_nr) = 657 image_offset(byte_nr / granularity / 8) * 8 + 658 (byte_nr / granularity) % 8 659 660If the size of the bitmap data is not a multiple of the cluster size then the 661last cluster of the bitmap data contains some unused tail bits. These bits must 662be zero. 663 664 665=== Dirty tracking bitmaps === 666 667Bitmaps with 'type' field equal to one are dirty tracking bitmaps. 668 669When the virtual disk is in use dirty tracking bitmap may be 'enabled' or 670'disabled'. While the bitmap is 'enabled', all writes to the virtual disk 671should be reflected in the bitmap. A set bit in the bitmap means that the 672corresponding range of the virtual disk (see above) was written to while the 673bitmap was 'enabled'. An unset bit means that this range was not written to. 674 675The software doesn't have to sync the bitmap in the image file with its 676representation in RAM after each write. Flag 'in_use' should be set while the 677bitmap is not synced. 678 679In the image file the 'enabled' state is reflected by the 'auto' flag. If this 680flag is set, the software must consider the bitmap as 'enabled' and start 681tracking virtual disk changes to this bitmap from the first write to the 682virtual disk. If this flag is not set then the bitmap is disabled. 683