qcow2.txt (39256B)
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 Note: backing files are incompatible with raw external data 29 files (auto-clear feature bit 1). 30 31 16 - 19: backing_file_size 32 Length of the backing file name in bytes. Must not be 33 longer than 1023 bytes. Undefined if the image doesn't have 34 a backing file. 35 36 20 - 23: cluster_bits 37 Number of bits that are used for addressing an offset 38 within a cluster (1 << cluster_bits is the cluster size). 39 Must not be less than 9 (i.e. 512 byte clusters). 40 41 Note: qemu as of today has an implementation limit of 2 MB 42 as the maximum cluster size and won't be able to open images 43 with larger cluster sizes. 44 45 Note: if the image has Extended L2 Entries then cluster_bits 46 must be at least 14 (i.e. 16384 byte clusters). 47 48 24 - 31: size 49 Virtual disk size in bytes. 50 51 Note: qemu has an implementation limit of 32 MB as 52 the maximum L1 table size. With a 2 MB cluster 53 size, it is unable to populate a virtual cluster 54 beyond 2 EB (61 bits); with a 512 byte cluster 55 size, it is unable to populate a virtual size 56 larger than 128 GB (37 bits). Meanwhile, L1/L2 57 table layouts limit an image to no more than 64 PB 58 (56 bits) of populated clusters, and an image may 59 hit other limits first (such as a file system's 60 maximum size). 61 62 32 - 35: crypt_method 63 0 for no encryption 64 1 for AES encryption 65 2 for LUKS encryption 66 67 36 - 39: l1_size 68 Number of entries in the active L1 table 69 70 40 - 47: l1_table_offset 71 Offset into the image file at which the active L1 table 72 starts. Must be aligned to a cluster boundary. 73 74 48 - 55: refcount_table_offset 75 Offset into the image file at which the refcount table 76 starts. Must be aligned to a cluster boundary. 77 78 56 - 59: refcount_table_clusters 79 Number of clusters that the refcount table occupies 80 81 60 - 63: nb_snapshots 82 Number of snapshots contained in the image 83 84 64 - 71: snapshots_offset 85 Offset into the image file at which the snapshot table 86 starts. Must be aligned to a cluster boundary. 87 88For version 2, the header is exactly 72 bytes in length, and finishes here. 89For version 3 or higher, the header length is at least 104 bytes, including 90the next fields through header_length. 91 92 72 - 79: incompatible_features 93 Bitmask of incompatible features. An implementation must 94 fail to open an image if an unknown bit is set. 95 96 Bit 0: Dirty bit. If this bit is set then refcounts 97 may be inconsistent, make sure to scan L1/L2 98 tables to repair refcounts before accessing the 99 image. 100 101 Bit 1: Corrupt bit. If this bit is set then any data 102 structure may be corrupt and the image must not 103 be written to (unless for regaining 104 consistency). 105 106 Bit 2: External data file bit. If this bit is set, an 107 external data file is used. Guest clusters are 108 then stored in the external data file. For such 109 images, clusters in the external data file are 110 not refcounted. The offset field in the 111 Standard Cluster Descriptor must match the 112 guest offset and neither compressed clusters 113 nor internal snapshots are supported. 114 115 An External Data File Name header extension may 116 be present if this bit is set. 117 118 Bit 3: Compression type bit. If this bit is set, 119 a non-default compression is used for compressed 120 clusters. The compression_type field must be 121 present and not zero. 122 123 Bit 4: Extended L2 Entries. If this bit is set then 124 L2 table entries use an extended format that 125 allows subcluster-based allocation. See the 126 Extended L2 Entries section for more details. 127 128 Bits 5-63: Reserved (set to 0) 129 130 80 - 87: compatible_features 131 Bitmask of compatible features. An implementation can 132 safely ignore any unknown bits that are set. 133 134 Bit 0: Lazy refcounts bit. If this bit is set then 135 lazy refcount updates can be used. This means 136 marking the image file dirty and postponing 137 refcount metadata updates. 138 139 Bits 1-63: Reserved (set to 0) 140 141 88 - 95: autoclear_features 142 Bitmask of auto-clear features. An implementation may only 143 write to an image with unknown auto-clear features if it 144 clears the respective bits from this field first. 145 146 Bit 0: Bitmaps extension bit 147 This bit indicates consistency for the bitmaps 148 extension data. 149 150 It is an error if this bit is set without the 151 bitmaps extension present. 152 153 If the bitmaps extension is present but this 154 bit is unset, the bitmaps extension data must be 155 considered inconsistent. 156 157 Bit 1: Raw external data bit 158 If this bit is set, the external data file can 159 be read as a consistent standalone raw image 160 without looking at the qcow2 metadata. 161 162 Setting this bit has a performance impact for 163 some operations on the image (e.g. writing 164 zeros requires writing to the data file instead 165 of only setting the zero flag in the L2 table 166 entry) and conflicts with backing files. 167 168 This bit may only be set if the External Data 169 File bit (incompatible feature bit 1) is also 170 set. 171 172 Bits 2-63: Reserved (set to 0) 173 174 96 - 99: refcount_order 175 Describes the width of a reference count block entry (width 176 in bits: refcount_bits = 1 << refcount_order). For version 2 177 images, the order is always assumed to be 4 178 (i.e. refcount_bits = 16). 179 This value may not exceed 6 (i.e. refcount_bits = 64). 180 181 100 - 103: header_length 182 Length of the header structure in bytes. For version 2 183 images, the length is always assumed to be 72 bytes. 184 For version 3 it's at least 104 bytes and must be a multiple 185 of 8. 186 187 188=== Additional fields (version 3 and higher) === 189 190In general, these fields are optional and may be safely ignored by the software, 191as well as filled by zeros (which is equal to field absence), if software needs 192to set field B, but does not care about field A which precedes B. More 193formally, additional fields have the following compatibility rules: 194 1951. If the value of the additional field must not be ignored for correct 196handling of the file, it will be accompanied by a corresponding incompatible 197feature bit. 198 1992. If there are no unrecognized incompatible feature bits set, an unknown 200additional field may be safely ignored other than preserving its value when 201rewriting the image header. 202 2033. An explicit value of 0 will have the same behavior as when the field is not 204present*, if not altered by a specific incompatible bit. 205 206*. A field is considered not present when header_length is less than or equal 207to the field's offset. Also, all additional fields are not present for 208version 2. 209 210 104: compression_type 211 212 Defines the compression method used for compressed clusters. 213 All compressed clusters in an image use the same compression 214 type. 215 216 If the incompatible bit "Compression type" is set: the field 217 must be present and non-zero (which means non-zlib 218 compression type). Otherwise, this field must not be present 219 or must be zero (which means zlib). 220 221 Available compression type values: 222 0: zlib <https://www.zlib.net/> 223 1: zstd <http://github.com/facebook/zstd> 224 225 226=== Header padding === 227 228@header_length must be a multiple of 8, which means that if the end of the last 229additional field is not aligned, some padding is needed. This padding must be 230zeroed, so that if some existing (or future) additional field will fall into 231the padding, it will be interpreted accordingly to point [3.] of the previous 232paragraph, i.e. in the same manner as when this field is not present. 233 234 235=== Header extensions === 236 237Directly after the image header, optional sections called header extensions can 238be stored. Each extension has a structure like the following: 239 240 Byte 0 - 3: Header extension type: 241 0x00000000 - End of the header extension area 242 0xe2792aca - Backing file format name string 243 0x6803f857 - Feature name table 244 0x23852875 - Bitmaps extension 245 0x0537be77 - Full disk encryption header pointer 246 0x44415441 - External data file name string 247 other - Unknown header extension, can be safely 248 ignored 249 250 4 - 7: Length of the header extension data 251 252 8 - n: Header extension data 253 254 n - m: Padding to round up the header extension size to the next 255 multiple of 8. 256 257Unless stated otherwise, each header extension type shall appear at most once 258in the same image. 259 260If the image has a backing file then the backing file name should be stored in 261the remaining space between the end of the header extension area and the end of 262the first cluster. It is not allowed to store other data here, so that an 263implementation can safely modify the header and add extensions without harming 264data of compatible features that it doesn't support. Compatible features that 265need space for additional data can use a header extension. 266 267 268== String header extensions == 269 270Some header extensions (such as the backing file format name and the external 271data file name) are just a single string. In this case, the header extension 272length is the string length and the string is not '\0' terminated. (The header 273extension padding can make it look like a string is '\0' terminated, but 274neither is padding always necessary nor is there a guarantee that zero bytes 275are used for padding.) 276 277 278== Feature name table == 279 280The feature name table is an optional header extension that contains the name 281for features used by the image. It can be used by applications that don't know 282the respective feature (e.g. because the feature was introduced only later) to 283display a useful error message. 284 285The number of entries in the feature name table is determined by the length of 286the header extension data. Each entry look like this: 287 288 Byte 0: Type of feature (select feature bitmap) 289 0: Incompatible feature 290 1: Compatible feature 291 2: Autoclear feature 292 293 1: Bit number within the selected feature bitmap (valid 294 values: 0-63) 295 296 2 - 47: Feature name (padded with zeros, but not necessarily null 297 terminated if it has full length) 298 299 300== Bitmaps extension == 301 302The bitmaps extension is an optional header extension. It provides the ability 303to store bitmaps related to a virtual disk. For now, there is only one bitmap 304type: the dirty tracking bitmap, which tracks virtual disk changes from some 305point in time. 306 307The data of the extension should be considered consistent only if the 308corresponding auto-clear feature bit is set, see autoclear_features above. 309 310The fields of the bitmaps extension are: 311 312 Byte 0 - 3: nb_bitmaps 313 The number of bitmaps contained in the image. Must be 314 greater than or equal to 1. 315 316 Note: Qemu currently only supports up to 65535 bitmaps per 317 image. 318 319 4 - 7: Reserved, must be zero. 320 321 8 - 15: bitmap_directory_size 322 Size of the bitmap directory in bytes. It is the cumulative 323 size of all (nb_bitmaps) bitmap directory entries. 324 325 16 - 23: bitmap_directory_offset 326 Offset into the image file at which the bitmap directory 327 starts. Must be aligned to a cluster boundary. 328 329== Full disk encryption header pointer == 330 331The full disk encryption header must be present if, and only if, the 332'crypt_method' header requires metadata. Currently this is only true 333of the 'LUKS' crypt method. The header extension must be absent for 334other methods. 335 336This header provides the offset at which the crypt method can store 337its additional data, as well as the length of such data. 338 339 Byte 0 - 7: Offset into the image file at which the encryption 340 header starts in bytes. Must be aligned to a cluster 341 boundary. 342 Byte 8 - 15: Length of the written encryption header in bytes. 343 Note actual space allocated in the qcow2 file may 344 be larger than this value, since it will be rounded 345 to the nearest multiple of the cluster size. Any 346 unused bytes in the allocated space will be initialized 347 to 0. 348 349For the LUKS crypt method, the encryption header works as follows. 350 351The first 592 bytes of the header clusters will contain the LUKS 352partition header. This is then followed by the key material data areas. 353The size of the key material data areas is determined by the number of 354stripes in the key slot and key size. Refer to the LUKS format 355specification ('docs/on-disk-format.pdf' in the cryptsetup source 356package) for details of the LUKS partition header format. 357 358In the LUKS partition header, the "payload-offset" field will be 359calculated as normal for the LUKS spec. ie the size of the LUKS 360header, plus key material regions, plus padding, relative to the 361start of the LUKS header. This offset value is not required to be 362qcow2 cluster aligned. Its value is currently never used in the 363context of qcow2, since the qcow2 file format itself defines where 364the real payload offset is, but none the less a valid payload offset 365should always be present. 366 367In the LUKS key slots header, the "key-material-offset" is relative 368to the start of the LUKS header clusters in the qcow2 container, 369not the start of the qcow2 file. 370 371Logically the layout looks like 372 373 +-----------------------------+ 374 | QCow2 header | 375 | QCow2 header extension X | 376 | QCow2 header extension FDE | 377 | QCow2 header extension ... | 378 | QCow2 header extension Z | 379 +-----------------------------+ 380 | ....other QCow2 tables.... | 381 . . 382 . . 383 +-----------------------------+ 384 | +-------------------------+ | 385 | | LUKS partition header | | 386 | +-------------------------+ | 387 | | LUKS key material 1 | | 388 | +-------------------------+ | 389 | | LUKS key material 2 | | 390 | +-------------------------+ | 391 | | LUKS key material ... | | 392 | +-------------------------+ | 393 | | LUKS key material 8 | | 394 | +-------------------------+ | 395 +-----------------------------+ 396 | QCow2 cluster payload | 397 . . 398 . . 399 . . 400 | | 401 +-----------------------------+ 402 403== Data encryption == 404 405When an encryption method is requested in the header, the image payload 406data must be encrypted/decrypted on every write/read. The image headers 407and metadata are never encrypted. 408 409The algorithms used for encryption vary depending on the method 410 411 - AES: 412 413 The AES cipher, in CBC mode, with 256 bit keys. 414 415 Initialization vectors generated using plain64 method, with 416 the virtual disk sector as the input tweak. 417 418 This format is no longer supported in QEMU system emulators, due 419 to a number of design flaws affecting its security. It is only 420 supported in the command line tools for the sake of back compatibility 421 and data liberation. 422 423 - LUKS: 424 425 The algorithms are specified in the LUKS header. 426 427 Initialization vectors generated using the method specified 428 in the LUKS header, with the physical disk sector as the 429 input tweak. 430 431== Host cluster management == 432 433qcow2 manages the allocation of host clusters by maintaining a reference count 434for each host cluster. A refcount of 0 means that the cluster is free, 1 means 435that it is used, and >= 2 means that it is used and any write access must 436perform a COW (copy on write) operation. 437 438The refcounts are managed in a two-level table. The first level is called 439refcount table and has a variable size (which is stored in the header). The 440refcount table can cover multiple clusters, however it needs to be contiguous 441in the image file. 442 443It contains pointers to the second level structures which are called refcount 444blocks and are exactly one cluster in size. 445 446Although a large enough refcount table can reserve clusters past 64 PB 447(56 bits) (assuming the underlying protocol can even be sized that 448large), note that some qcow2 metadata such as L1/L2 tables must point 449to clusters prior to that point. 450 451Note: qemu has an implementation limit of 8 MB as the maximum refcount 452table size. With a 2 MB cluster size and a default refcount_order of 4534, it is unable to reference host resources beyond 2 EB (61 bits); in 454the worst case, with a 512 cluster size and refcount_order of 6, it is 455unable to access beyond 32 GB (35 bits). 456 457Given an offset into the image file, the refcount of its cluster can be 458obtained as follows: 459 460 refcount_block_entries = (cluster_size * 8 / refcount_bits) 461 462 refcount_block_index = (offset / cluster_size) % refcount_block_entries 463 refcount_table_index = (offset / cluster_size) / refcount_block_entries 464 465 refcount_block = load_cluster(refcount_table[refcount_table_index]); 466 return refcount_block[refcount_block_index]; 467 468Refcount table entry: 469 470 Bit 0 - 8: Reserved (set to 0) 471 472 9 - 63: Bits 9-63 of the offset into the image file at which the 473 refcount block starts. Must be aligned to a cluster 474 boundary. 475 476 If this is 0, the corresponding refcount block has not yet 477 been allocated. All refcounts managed by this refcount block 478 are 0. 479 480Refcount block entry (x = refcount_bits - 1): 481 482 Bit 0 - x: Reference count of the cluster. If refcount_bits implies a 483 sub-byte width, note that bit 0 means the least significant 484 bit in this context. 485 486 487== Cluster mapping == 488 489Just as for refcounts, qcow2 uses a two-level structure for the mapping of 490guest clusters to host clusters. They are called L1 and L2 table. 491 492The L1 table has a variable size (stored in the header) and may use multiple 493clusters, however it must be contiguous in the image file. L2 tables are 494exactly one cluster in size. 495 496The L1 and L2 tables have implications on the maximum virtual file 497size; for a given L1 table size, a larger cluster size is required for 498the guest to have access to more space. Furthermore, a virtual 499cluster must currently map to a host offset below 64 PB (56 bits) 500(although this limit could be relaxed by putting reserved bits into 501use). Additionally, as cluster size increases, the maximum host 502offset for a compressed cluster is reduced (a 2M cluster size requires 503compressed clusters to reside below 512 TB (49 bits), and this limit 504cannot be relaxed without an incompatible layout change). 505 506Given an offset into the virtual disk, the offset into the image file can be 507obtained as follows: 508 509 l2_entries = (cluster_size / sizeof(uint64_t)) [*] 510 511 l2_index = (offset / cluster_size) % l2_entries 512 l1_index = (offset / cluster_size) / l2_entries 513 514 l2_table = load_cluster(l1_table[l1_index]); 515 cluster_offset = l2_table[l2_index]; 516 517 return cluster_offset + (offset % cluster_size) 518 519 [*] this changes if Extended L2 Entries are enabled, see next section 520 521L1 table entry: 522 523 Bit 0 - 8: Reserved (set to 0) 524 525 9 - 55: Bits 9-55 of the offset into the image file at which the L2 526 table starts. Must be aligned to a cluster boundary. If the 527 offset is 0, the L2 table and all clusters described by this 528 L2 table are unallocated. 529 530 56 - 62: Reserved (set to 0) 531 532 63: 0 for an L2 table that is unused or requires COW, 1 if its 533 refcount is exactly one. This information is only accurate 534 in the active L1 table. 535 536L2 table entry: 537 538 Bit 0 - 61: Cluster descriptor 539 540 62: 0 for standard clusters 541 1 for compressed clusters 542 543 63: 0 for clusters that are unused, compressed or require COW. 544 1 for standard clusters whose refcount is exactly one. 545 This information is only accurate in L2 tables 546 that are reachable from the active L1 table. 547 548 With external data files, all guest clusters have an 549 implicit refcount of 1 (because of the fixed host = guest 550 mapping for guest cluster offsets), so this bit should be 1 551 for all allocated clusters. 552 553Standard Cluster Descriptor: 554 555 Bit 0: If set to 1, the cluster reads as all zeros. The host 556 cluster offset can be used to describe a preallocation, 557 but it won't be used for reading data from this cluster, 558 nor is data read from the backing file if the cluster is 559 unallocated. 560 561 With version 2 or with extended L2 entries (see the next 562 section), this is always 0. 563 564 1 - 8: Reserved (set to 0) 565 566 9 - 55: Bits 9-55 of host cluster offset. Must be aligned to a 567 cluster boundary. If the offset is 0 and bit 63 is clear, 568 the cluster is unallocated. The offset may only be 0 with 569 bit 63 set (indicating a host cluster offset of 0) when an 570 external data file is used. 571 572 56 - 61: Reserved (set to 0) 573 574 575Compressed Clusters Descriptor (x = 62 - (cluster_bits - 8)): 576 577 Bit 0 - x-1: Host cluster offset. This is usually _not_ aligned to a 578 cluster or sector boundary! If cluster_bits is 579 small enough that this field includes bits beyond 580 55, those upper bits must be set to 0. 581 582 x - 61: Number of additional 512-byte sectors used for the 583 compressed data, beyond the sector containing the offset 584 in the previous field. Some of these sectors may reside 585 in the next contiguous host cluster. 586 587 Note that the compressed data does not necessarily occupy 588 all of the bytes in the final sector; rather, decompression 589 stops when it has produced a cluster of data. 590 591 Another compressed cluster may map to the tail of the final 592 sector used by this compressed cluster. 593 594If a cluster is unallocated, read requests shall read the data from the backing 595file (except if bit 0 in the Standard Cluster Descriptor is set). If there is 596no backing file or the backing file is smaller than the image, they shall read 597zeros for all parts that are not covered by the backing file. 598 599== Extended L2 Entries == 600 601An image uses Extended L2 Entries if bit 4 is set on the incompatible_features 602field of the header. 603 604In these images standard data clusters are divided into 32 subclusters of the 605same size. They are contiguous and start from the beginning of the cluster. 606Subclusters can be allocated independently and the L2 entry contains information 607indicating the status of each one of them. Compressed data clusters don't have 608subclusters so they are treated the same as in images without this feature. 609 610The size of an extended L2 entry is 128 bits so the number of entries per table 611is calculated using this formula: 612 613 l2_entries = (cluster_size / (2 * sizeof(uint64_t))) 614 615The first 64 bits have the same format as the standard L2 table entry described 616in the previous section, with the exception of bit 0 of the standard cluster 617descriptor. 618 619The last 64 bits contain a subcluster allocation bitmap with this format: 620 621Subcluster Allocation Bitmap (for standard clusters): 622 623 Bit 0 - 31: Allocation status (one bit per subcluster) 624 625 1: the subcluster is allocated. In this case the 626 host cluster offset field must contain a valid 627 offset. 628 0: the subcluster is not allocated. In this case 629 read requests shall go to the backing file or 630 return zeros if there is no backing file data. 631 632 Bits are assigned starting from the least significant 633 one (i.e. bit x is used for subcluster x). 634 635 32 - 63 Subcluster reads as zeros (one bit per subcluster) 636 637 1: the subcluster reads as zeros. In this case the 638 allocation status bit must be unset. The host 639 cluster offset field may or may not be set. 640 0: no effect. 641 642 Bits are assigned starting from the least significant 643 one (i.e. bit x is used for subcluster x - 32). 644 645Subcluster Allocation Bitmap (for compressed clusters): 646 647 Bit 0 - 63: Reserved (set to 0) 648 Compressed clusters don't have subclusters, 649 so this field is not used. 650 651== Snapshots == 652 653qcow2 supports internal snapshots. Their basic principle of operation is to 654switch the active L1 table, so that a different set of host clusters are 655exposed to the guest. 656 657When creating a snapshot, the L1 table should be copied and the refcount of all 658L2 tables and clusters reachable from this L1 table must be increased, so that 659a write causes a COW and isn't visible in other snapshots. 660 661When loading a snapshot, bit 63 of all entries in the new active L1 table and 662all L2 tables referenced by it must be reconstructed from the refcount table 663as it doesn't need to be accurate in inactive L1 tables. 664 665A directory of all snapshots is stored in the snapshot table, a contiguous area 666in the image file, whose starting offset and length are given by the header 667fields snapshots_offset and nb_snapshots. The entries of the snapshot table 668have variable length, depending on the length of ID, name and extra data. 669 670Snapshot table entry: 671 672 Byte 0 - 7: Offset into the image file at which the L1 table for the 673 snapshot starts. Must be aligned to a cluster boundary. 674 675 8 - 11: Number of entries in the L1 table of the snapshots 676 677 12 - 13: Length of the unique ID string describing the snapshot 678 679 14 - 15: Length of the name of the snapshot 680 681 16 - 19: Time at which the snapshot was taken in seconds since the 682 Epoch 683 684 20 - 23: Subsecond part of the time at which the snapshot was taken 685 in nanoseconds 686 687 24 - 31: Time that the guest was running until the snapshot was 688 taken in nanoseconds 689 690 32 - 35: Size of the VM state in bytes. 0 if no VM state is saved. 691 If there is VM state, it starts at the first cluster 692 described by first L1 table entry that doesn't describe a 693 regular guest cluster (i.e. VM state is stored like guest 694 disk content, except that it is stored at offsets that are 695 larger than the virtual disk presented to the guest) 696 697 36 - 39: Size of extra data in the table entry (used for future 698 extensions of the format) 699 700 variable: Extra data for future extensions. Unknown fields must be 701 ignored. Currently defined are (offset relative to snapshot 702 table entry): 703 704 Byte 40 - 47: Size of the VM state in bytes. 0 if no VM 705 state is saved. If this field is present, 706 the 32-bit value in bytes 32-35 is ignored. 707 708 Byte 48 - 55: Virtual disk size of the snapshot in bytes 709 710 Byte 56 - 63: icount value which corresponds to 711 the record/replay instruction count 712 when the snapshot was taken. Set to -1 713 if icount was disabled 714 715 Version 3 images must include extra data at least up to 716 byte 55. 717 718 variable: Unique ID string for the snapshot (not null terminated) 719 720 variable: Name of the snapshot (not null terminated) 721 722 variable: Padding to round up the snapshot table entry size to the 723 next multiple of 8. 724 725 726== Bitmaps == 727 728As mentioned above, the bitmaps extension provides the ability to store bitmaps 729related to a virtual disk. This section describes how these bitmaps are stored. 730 731All stored bitmaps are related to the virtual disk stored in the same image, so 732each bitmap size is equal to the virtual disk size. 733 734Each bit of the bitmap is responsible for strictly defined range of the virtual 735disk. For bit number bit_nr the corresponding range (in bytes) will be: 736 737 [bit_nr * bitmap_granularity .. (bit_nr + 1) * bitmap_granularity - 1] 738 739Granularity is a property of the concrete bitmap, see below. 740 741 742=== Bitmap directory === 743 744Each bitmap saved in the image is described in a bitmap directory entry. The 745bitmap directory is a contiguous area in the image file, whose starting offset 746and length are given by the header extension fields bitmap_directory_offset and 747bitmap_directory_size. The entries of the bitmap directory have variable 748length, depending on the lengths of the bitmap name and extra data. 749 750Structure of a bitmap directory entry: 751 752 Byte 0 - 7: bitmap_table_offset 753 Offset into the image file at which the bitmap table 754 (described below) for the bitmap starts. Must be aligned to 755 a cluster boundary. 756 757 8 - 11: bitmap_table_size 758 Number of entries in the bitmap table of the bitmap. 759 760 12 - 15: flags 761 Bit 762 0: in_use 763 The bitmap was not saved correctly and may be 764 inconsistent. Although the bitmap metadata is still 765 well-formed from a qcow2 perspective, the metadata 766 (such as the auto flag or bitmap size) or data 767 contents may be outdated. 768 769 1: auto 770 The bitmap must reflect all changes of the virtual 771 disk by any application that would write to this qcow2 772 file (including writes, snapshot switching, etc.). The 773 type of this bitmap must be 'dirty tracking bitmap'. 774 775 2: extra_data_compatible 776 This flags is meaningful when the extra data is 777 unknown to the software (currently any extra data is 778 unknown to Qemu). 779 If it is set, the bitmap may be used as expected, extra 780 data must be left as is. 781 If it is not set, the bitmap must not be used, but 782 both it and its extra data be left as is. 783 784 Bits 3 - 31 are reserved and must be 0. 785 786 16: type 787 This field describes the sort of the bitmap. 788 Values: 789 1: Dirty tracking bitmap 790 791 Values 0, 2 - 255 are reserved. 792 793 17: granularity_bits 794 Granularity bits. Valid values: 0 - 63. 795 796 Note: Qemu currently supports only values 9 - 31. 797 798 Granularity is calculated as 799 granularity = 1 << granularity_bits 800 801 A bitmap's granularity is how many bytes of the image 802 accounts for one bit of the bitmap. 803 804 18 - 19: name_size 805 Size of the bitmap name. Must be non-zero. 806 807 Note: Qemu currently doesn't support values greater than 808 1023. 809 810 20 - 23: extra_data_size 811 Size of type-specific extra data. 812 813 For now, as no extra data is defined, extra_data_size is 814 reserved and should be zero. If it is non-zero the 815 behavior is defined by extra_data_compatible flag. 816 817 variable: extra_data 818 Extra data for the bitmap, occupying extra_data_size bytes. 819 Extra data must never contain references to clusters or in 820 some other way allocate additional clusters. 821 822 variable: name 823 The name of the bitmap (not null terminated), occupying 824 name_size bytes. Must be unique among all bitmap names 825 within the bitmaps extension. 826 827 variable: Padding to round up the bitmap directory entry size to the 828 next multiple of 8. All bytes of the padding must be zero. 829 830 831=== Bitmap table === 832 833Each bitmap is stored using a one-level structure (as opposed to two-level 834structures like for refcounts and guest clusters mapping) for the mapping of 835bitmap data to host clusters. This structure is called the bitmap table. 836 837Each bitmap table has a variable size (stored in the bitmap directory entry) 838and may use multiple clusters, however, it must be contiguous in the image 839file. 840 841Structure of a bitmap table entry: 842 843 Bit 0: Reserved and must be zero if bits 9 - 55 are non-zero. 844 If bits 9 - 55 are zero: 845 0: Cluster should be read as all zeros. 846 1: Cluster should be read as all ones. 847 848 1 - 8: Reserved and must be zero. 849 850 9 - 55: Bits 9 - 55 of the host cluster offset. Must be aligned to 851 a cluster boundary. If the offset is 0, the cluster is 852 unallocated; in that case, bit 0 determines how this 853 cluster should be treated during reads. 854 855 56 - 63: Reserved and must be zero. 856 857 858=== Bitmap data === 859 860As noted above, bitmap data is stored in separate clusters, described by the 861bitmap table. Given an offset (in bytes) into the bitmap data, the offset into 862the image file can be obtained as follows: 863 864 image_offset(bitmap_data_offset) = 865 bitmap_table[bitmap_data_offset / cluster_size] + 866 (bitmap_data_offset % cluster_size) 867 868This offset is not defined if bits 9 - 55 of bitmap table entry are zero (see 869above). 870 871Given an offset byte_nr into the virtual disk and the bitmap's granularity, the 872bit offset into the image file to the corresponding bit of the bitmap can be 873calculated like this: 874 875 bit_offset(byte_nr) = 876 image_offset(byte_nr / granularity / 8) * 8 + 877 (byte_nr / granularity) % 8 878 879If the size of the bitmap data is not a multiple of the cluster size then the 880last cluster of the bitmap data contains some unused tail bits. These bits must 881be zero. 882 883 884=== Dirty tracking bitmaps === 885 886Bitmaps with 'type' field equal to one are dirty tracking bitmaps. 887 888When the virtual disk is in use dirty tracking bitmap may be 'enabled' or 889'disabled'. While the bitmap is 'enabled', all writes to the virtual disk 890should be reflected in the bitmap. A set bit in the bitmap means that the 891corresponding range of the virtual disk (see above) was written to while the 892bitmap was 'enabled'. An unset bit means that this range was not written to. 893 894The software doesn't have to sync the bitmap in the image file with its 895representation in RAM after each write or metadata change. Flag 'in_use' 896should be set while the bitmap is not synced. 897 898In the image file the 'enabled' state is reflected by the 'auto' flag. If this 899flag is set, the software must consider the bitmap as 'enabled' and start 900tracking virtual disk changes to this bitmap from the first write to the 901virtual disk. If this flag is not set then the bitmap is disabled.