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-deflate 218 compression type). Otherwise, this field must not be present 219 or must be zero (which means deflate). 220 221 Available compression type values: 222 0: deflate <https://www.ietf.org/rfc/rfc1951.txt> 223 1: zstd <http://github.com/facebook/zstd> 224 225 The deflate compression type is called "zlib" 226 <https://www.zlib.net/> in QEMU. However, clusters with the 227 deflate compression type do not have zlib headers. 228 229 230=== Header padding === 231 232@header_length must be a multiple of 8, which means that if the end of the last 233additional field is not aligned, some padding is needed. This padding must be 234zeroed, so that if some existing (or future) additional field will fall into 235the padding, it will be interpreted accordingly to point [3.] of the previous 236paragraph, i.e. in the same manner as when this field is not present. 237 238 239=== Header extensions === 240 241Directly after the image header, optional sections called header extensions can 242be stored. Each extension has a structure like the following: 243 244 Byte 0 - 3: Header extension type: 245 0x00000000 - End of the header extension area 246 0xe2792aca - Backing file format name string 247 0x6803f857 - Feature name table 248 0x23852875 - Bitmaps extension 249 0x0537be77 - Full disk encryption header pointer 250 0x44415441 - External data file name string 251 other - Unknown header extension, can be safely 252 ignored 253 254 4 - 7: Length of the header extension data 255 256 8 - n: Header extension data 257 258 n - m: Padding to round up the header extension size to the next 259 multiple of 8. 260 261Unless stated otherwise, each header extension type shall appear at most once 262in the same image. 263 264If the image has a backing file then the backing file name should be stored in 265the remaining space between the end of the header extension area and the end of 266the first cluster. It is not allowed to store other data here, so that an 267implementation can safely modify the header and add extensions without harming 268data of compatible features that it doesn't support. Compatible features that 269need space for additional data can use a header extension. 270 271 272== String header extensions == 273 274Some header extensions (such as the backing file format name and the external 275data file name) are just a single string. In this case, the header extension 276length is the string length and the string is not '\0' terminated. (The header 277extension padding can make it look like a string is '\0' terminated, but 278neither is padding always necessary nor is there a guarantee that zero bytes 279are used for padding.) 280 281 282== Feature name table == 283 284The feature name table is an optional header extension that contains the name 285for features used by the image. It can be used by applications that don't know 286the respective feature (e.g. because the feature was introduced only later) to 287display a useful error message. 288 289The number of entries in the feature name table is determined by the length of 290the header extension data. Each entry look like this: 291 292 Byte 0: Type of feature (select feature bitmap) 293 0: Incompatible feature 294 1: Compatible feature 295 2: Autoclear feature 296 297 1: Bit number within the selected feature bitmap (valid 298 values: 0-63) 299 300 2 - 47: Feature name (padded with zeros, but not necessarily null 301 terminated if it has full length) 302 303 304== Bitmaps extension == 305 306The bitmaps extension is an optional header extension. It provides the ability 307to store bitmaps related to a virtual disk. For now, there is only one bitmap 308type: the dirty tracking bitmap, which tracks virtual disk changes from some 309point in time. 310 311The data of the extension should be considered consistent only if the 312corresponding auto-clear feature bit is set, see autoclear_features above. 313 314The fields of the bitmaps extension are: 315 316 Byte 0 - 3: nb_bitmaps 317 The number of bitmaps contained in the image. Must be 318 greater than or equal to 1. 319 320 Note: QEMU currently only supports up to 65535 bitmaps per 321 image. 322 323 4 - 7: Reserved, must be zero. 324 325 8 - 15: bitmap_directory_size 326 Size of the bitmap directory in bytes. It is the cumulative 327 size of all (nb_bitmaps) bitmap directory entries. 328 329 16 - 23: bitmap_directory_offset 330 Offset into the image file at which the bitmap directory 331 starts. Must be aligned to a cluster boundary. 332 333== Full disk encryption header pointer == 334 335The full disk encryption header must be present if, and only if, the 336'crypt_method' header requires metadata. Currently this is only true 337of the 'LUKS' crypt method. The header extension must be absent for 338other methods. 339 340This header provides the offset at which the crypt method can store 341its additional data, as well as the length of such data. 342 343 Byte 0 - 7: Offset into the image file at which the encryption 344 header starts in bytes. Must be aligned to a cluster 345 boundary. 346 Byte 8 - 15: Length of the written encryption header in bytes. 347 Note actual space allocated in the qcow2 file may 348 be larger than this value, since it will be rounded 349 to the nearest multiple of the cluster size. Any 350 unused bytes in the allocated space will be initialized 351 to 0. 352 353For the LUKS crypt method, the encryption header works as follows. 354 355The first 592 bytes of the header clusters will contain the LUKS 356partition header. This is then followed by the key material data areas. 357The size of the key material data areas is determined by the number of 358stripes in the key slot and key size. Refer to the LUKS format 359specification ('docs/on-disk-format.pdf' in the cryptsetup source 360package) for details of the LUKS partition header format. 361 362In the LUKS partition header, the "payload-offset" field will be 363calculated as normal for the LUKS spec. ie the size of the LUKS 364header, plus key material regions, plus padding, relative to the 365start of the LUKS header. This offset value is not required to be 366qcow2 cluster aligned. Its value is currently never used in the 367context of qcow2, since the qcow2 file format itself defines where 368the real payload offset is, but none the less a valid payload offset 369should always be present. 370 371In the LUKS key slots header, the "key-material-offset" is relative 372to the start of the LUKS header clusters in the qcow2 container, 373not the start of the qcow2 file. 374 375Logically the layout looks like 376 377 +-----------------------------+ 378 | QCow2 header | 379 | QCow2 header extension X | 380 | QCow2 header extension FDE | 381 | QCow2 header extension ... | 382 | QCow2 header extension Z | 383 +-----------------------------+ 384 | ....other QCow2 tables.... | 385 . . 386 . . 387 +-----------------------------+ 388 | +-------------------------+ | 389 | | LUKS partition header | | 390 | +-------------------------+ | 391 | | LUKS key material 1 | | 392 | +-------------------------+ | 393 | | LUKS key material 2 | | 394 | +-------------------------+ | 395 | | LUKS key material ... | | 396 | +-------------------------+ | 397 | | LUKS key material 8 | | 398 | +-------------------------+ | 399 +-----------------------------+ 400 | QCow2 cluster payload | 401 . . 402 . . 403 . . 404 | | 405 +-----------------------------+ 406 407== Data encryption == 408 409When an encryption method is requested in the header, the image payload 410data must be encrypted/decrypted on every write/read. The image headers 411and metadata are never encrypted. 412 413The algorithms used for encryption vary depending on the method 414 415 - AES: 416 417 The AES cipher, in CBC mode, with 256 bit keys. 418 419 Initialization vectors generated using plain64 method, with 420 the virtual disk sector as the input tweak. 421 422 This format is no longer supported in QEMU system emulators, due 423 to a number of design flaws affecting its security. It is only 424 supported in the command line tools for the sake of back compatibility 425 and data liberation. 426 427 - LUKS: 428 429 The algorithms are specified in the LUKS header. 430 431 Initialization vectors generated using the method specified 432 in the LUKS header, with the physical disk sector as the 433 input tweak. 434 435== Host cluster management == 436 437qcow2 manages the allocation of host clusters by maintaining a reference count 438for each host cluster. A refcount of 0 means that the cluster is free, 1 means 439that it is used, and >= 2 means that it is used and any write access must 440perform a COW (copy on write) operation. 441 442The refcounts are managed in a two-level table. The first level is called 443refcount table and has a variable size (which is stored in the header). The 444refcount table can cover multiple clusters, however it needs to be contiguous 445in the image file. 446 447It contains pointers to the second level structures which are called refcount 448blocks and are exactly one cluster in size. 449 450Although a large enough refcount table can reserve clusters past 64 PB 451(56 bits) (assuming the underlying protocol can even be sized that 452large), note that some qcow2 metadata such as L1/L2 tables must point 453to clusters prior to that point. 454 455Note: qemu has an implementation limit of 8 MB as the maximum refcount 456table size. With a 2 MB cluster size and a default refcount_order of 4574, it is unable to reference host resources beyond 2 EB (61 bits); in 458the worst case, with a 512 cluster size and refcount_order of 6, it is 459unable to access beyond 32 GB (35 bits). 460 461Given an offset into the image file, the refcount of its cluster can be 462obtained as follows: 463 464 refcount_block_entries = (cluster_size * 8 / refcount_bits) 465 466 refcount_block_index = (offset / cluster_size) % refcount_block_entries 467 refcount_table_index = (offset / cluster_size) / refcount_block_entries 468 469 refcount_block = load_cluster(refcount_table[refcount_table_index]); 470 return refcount_block[refcount_block_index]; 471 472Refcount table entry: 473 474 Bit 0 - 8: Reserved (set to 0) 475 476 9 - 63: Bits 9-63 of the offset into the image file at which the 477 refcount block starts. Must be aligned to a cluster 478 boundary. 479 480 If this is 0, the corresponding refcount block has not yet 481 been allocated. All refcounts managed by this refcount block 482 are 0. 483 484Refcount block entry (x = refcount_bits - 1): 485 486 Bit 0 - x: Reference count of the cluster. If refcount_bits implies a 487 sub-byte width, note that bit 0 means the least significant 488 bit in this context. 489 490 491== Cluster mapping == 492 493Just as for refcounts, qcow2 uses a two-level structure for the mapping of 494guest clusters to host clusters. They are called L1 and L2 table. 495 496The L1 table has a variable size (stored in the header) and may use multiple 497clusters, however it must be contiguous in the image file. L2 tables are 498exactly one cluster in size. 499 500The L1 and L2 tables have implications on the maximum virtual file 501size; for a given L1 table size, a larger cluster size is required for 502the guest to have access to more space. Furthermore, a virtual 503cluster must currently map to a host offset below 64 PB (56 bits) 504(although this limit could be relaxed by putting reserved bits into 505use). Additionally, as cluster size increases, the maximum host 506offset for a compressed cluster is reduced (a 2M cluster size requires 507compressed clusters to reside below 512 TB (49 bits), and this limit 508cannot be relaxed without an incompatible layout change). 509 510Given an offset into the virtual disk, the offset into the image file can be 511obtained as follows: 512 513 l2_entries = (cluster_size / sizeof(uint64_t)) [*] 514 515 l2_index = (offset / cluster_size) % l2_entries 516 l1_index = (offset / cluster_size) / l2_entries 517 518 l2_table = load_cluster(l1_table[l1_index]); 519 cluster_offset = l2_table[l2_index]; 520 521 return cluster_offset + (offset % cluster_size) 522 523 [*] this changes if Extended L2 Entries are enabled, see next section 524 525L1 table entry: 526 527 Bit 0 - 8: Reserved (set to 0) 528 529 9 - 55: Bits 9-55 of the offset into the image file at which the L2 530 table starts. Must be aligned to a cluster boundary. If the 531 offset is 0, the L2 table and all clusters described by this 532 L2 table are unallocated. 533 534 56 - 62: Reserved (set to 0) 535 536 63: 0 for an L2 table that is unused or requires COW, 1 if its 537 refcount is exactly one. This information is only accurate 538 in the active L1 table. 539 540L2 table entry: 541 542 Bit 0 - 61: Cluster descriptor 543 544 62: 0 for standard clusters 545 1 for compressed clusters 546 547 63: 0 for clusters that are unused, compressed or require COW. 548 1 for standard clusters whose refcount is exactly one. 549 This information is only accurate in L2 tables 550 that are reachable from the active L1 table. 551 552 With external data files, all guest clusters have an 553 implicit refcount of 1 (because of the fixed host = guest 554 mapping for guest cluster offsets), so this bit should be 1 555 for all allocated clusters. 556 557Standard Cluster Descriptor: 558 559 Bit 0: If set to 1, the cluster reads as all zeros. The host 560 cluster offset can be used to describe a preallocation, 561 but it won't be used for reading data from this cluster, 562 nor is data read from the backing file if the cluster is 563 unallocated. 564 565 With version 2 or with extended L2 entries (see the next 566 section), this is always 0. 567 568 1 - 8: Reserved (set to 0) 569 570 9 - 55: Bits 9-55 of host cluster offset. Must be aligned to a 571 cluster boundary. If the offset is 0 and bit 63 is clear, 572 the cluster is unallocated. The offset may only be 0 with 573 bit 63 set (indicating a host cluster offset of 0) when an 574 external data file is used. 575 576 56 - 61: Reserved (set to 0) 577 578 579Compressed Clusters Descriptor (x = 62 - (cluster_bits - 8)): 580 581 Bit 0 - x-1: Host cluster offset. This is usually _not_ aligned to a 582 cluster or sector boundary! If cluster_bits is 583 small enough that this field includes bits beyond 584 55, those upper bits must be set to 0. 585 586 x - 61: Number of additional 512-byte sectors used for the 587 compressed data, beyond the sector containing the offset 588 in the previous field. Some of these sectors may reside 589 in the next contiguous host cluster. 590 591 Note that the compressed data does not necessarily occupy 592 all of the bytes in the final sector; rather, decompression 593 stops when it has produced a cluster of data. 594 595 Another compressed cluster may map to the tail of the final 596 sector used by this compressed cluster. 597 598If a cluster is unallocated, read requests shall read the data from the backing 599file (except if bit 0 in the Standard Cluster Descriptor is set). If there is 600no backing file or the backing file is smaller than the image, they shall read 601zeros for all parts that are not covered by the backing file. 602 603== Extended L2 Entries == 604 605An image uses Extended L2 Entries if bit 4 is set on the incompatible_features 606field of the header. 607 608In these images standard data clusters are divided into 32 subclusters of the 609same size. They are contiguous and start from the beginning of the cluster. 610Subclusters can be allocated independently and the L2 entry contains information 611indicating the status of each one of them. Compressed data clusters don't have 612subclusters so they are treated the same as in images without this feature. 613 614The size of an extended L2 entry is 128 bits so the number of entries per table 615is calculated using this formula: 616 617 l2_entries = (cluster_size / (2 * sizeof(uint64_t))) 618 619The first 64 bits have the same format as the standard L2 table entry described 620in the previous section, with the exception of bit 0 of the standard cluster 621descriptor. 622 623The last 64 bits contain a subcluster allocation bitmap with this format: 624 625Subcluster Allocation Bitmap (for standard clusters): 626 627 Bit 0 - 31: Allocation status (one bit per subcluster) 628 629 1: the subcluster is allocated. In this case the 630 host cluster offset field must contain a valid 631 offset. 632 0: the subcluster is not allocated. In this case 633 read requests shall go to the backing file or 634 return zeros if there is no backing file data. 635 636 Bits are assigned starting from the least significant 637 one (i.e. bit x is used for subcluster x). 638 639 32 - 63 Subcluster reads as zeros (one bit per subcluster) 640 641 1: the subcluster reads as zeros. In this case the 642 allocation status bit must be unset. The host 643 cluster offset field may or may not be set. 644 0: no effect. 645 646 Bits are assigned starting from the least significant 647 one (i.e. bit x is used for subcluster x - 32). 648 649Subcluster Allocation Bitmap (for compressed clusters): 650 651 Bit 0 - 63: Reserved (set to 0) 652 Compressed clusters don't have subclusters, 653 so this field is not used. 654 655== Snapshots == 656 657qcow2 supports internal snapshots. Their basic principle of operation is to 658switch the active L1 table, so that a different set of host clusters are 659exposed to the guest. 660 661When creating a snapshot, the L1 table should be copied and the refcount of all 662L2 tables and clusters reachable from this L1 table must be increased, so that 663a write causes a COW and isn't visible in other snapshots. 664 665When loading a snapshot, bit 63 of all entries in the new active L1 table and 666all L2 tables referenced by it must be reconstructed from the refcount table 667as it doesn't need to be accurate in inactive L1 tables. 668 669A directory of all snapshots is stored in the snapshot table, a contiguous area 670in the image file, whose starting offset and length are given by the header 671fields snapshots_offset and nb_snapshots. The entries of the snapshot table 672have variable length, depending on the length of ID, name and extra data. 673 674Snapshot table entry: 675 676 Byte 0 - 7: Offset into the image file at which the L1 table for the 677 snapshot starts. Must be aligned to a cluster boundary. 678 679 8 - 11: Number of entries in the L1 table of the snapshots 680 681 12 - 13: Length of the unique ID string describing the snapshot 682 683 14 - 15: Length of the name of the snapshot 684 685 16 - 19: Time at which the snapshot was taken in seconds since the 686 Epoch 687 688 20 - 23: Subsecond part of the time at which the snapshot was taken 689 in nanoseconds 690 691 24 - 31: Time that the guest was running until the snapshot was 692 taken in nanoseconds 693 694 32 - 35: Size of the VM state in bytes. 0 if no VM state is saved. 695 If there is VM state, it starts at the first cluster 696 described by first L1 table entry that doesn't describe a 697 regular guest cluster (i.e. VM state is stored like guest 698 disk content, except that it is stored at offsets that are 699 larger than the virtual disk presented to the guest) 700 701 36 - 39: Size of extra data in the table entry (used for future 702 extensions of the format) 703 704 variable: Extra data for future extensions. Unknown fields must be 705 ignored. Currently defined are (offset relative to snapshot 706 table entry): 707 708 Byte 40 - 47: Size of the VM state in bytes. 0 if no VM 709 state is saved. If this field is present, 710 the 32-bit value in bytes 32-35 is ignored. 711 712 Byte 48 - 55: Virtual disk size of the snapshot in bytes 713 714 Byte 56 - 63: icount value which corresponds to 715 the record/replay instruction count 716 when the snapshot was taken. Set to -1 717 if icount was disabled 718 719 Version 3 images must include extra data at least up to 720 byte 55. 721 722 variable: Unique ID string for the snapshot (not null terminated) 723 724 variable: Name of the snapshot (not null terminated) 725 726 variable: Padding to round up the snapshot table entry size to the 727 next multiple of 8. 728 729 730== Bitmaps == 731 732As mentioned above, the bitmaps extension provides the ability to store bitmaps 733related to a virtual disk. This section describes how these bitmaps are stored. 734 735All stored bitmaps are related to the virtual disk stored in the same image, so 736each bitmap size is equal to the virtual disk size. 737 738Each bit of the bitmap is responsible for strictly defined range of the virtual 739disk. For bit number bit_nr the corresponding range (in bytes) will be: 740 741 [bit_nr * bitmap_granularity .. (bit_nr + 1) * bitmap_granularity - 1] 742 743Granularity is a property of the concrete bitmap, see below. 744 745 746=== Bitmap directory === 747 748Each bitmap saved in the image is described in a bitmap directory entry. The 749bitmap directory is a contiguous area in the image file, whose starting offset 750and length are given by the header extension fields bitmap_directory_offset and 751bitmap_directory_size. The entries of the bitmap directory have variable 752length, depending on the lengths of the bitmap name and extra data. 753 754Structure of a bitmap directory entry: 755 756 Byte 0 - 7: bitmap_table_offset 757 Offset into the image file at which the bitmap table 758 (described below) for the bitmap starts. Must be aligned to 759 a cluster boundary. 760 761 8 - 11: bitmap_table_size 762 Number of entries in the bitmap table of the bitmap. 763 764 12 - 15: flags 765 Bit 766 0: in_use 767 The bitmap was not saved correctly and may be 768 inconsistent. Although the bitmap metadata is still 769 well-formed from a qcow2 perspective, the metadata 770 (such as the auto flag or bitmap size) or data 771 contents may be outdated. 772 773 1: auto 774 The bitmap must reflect all changes of the virtual 775 disk by any application that would write to this qcow2 776 file (including writes, snapshot switching, etc.). The 777 type of this bitmap must be 'dirty tracking bitmap'. 778 779 2: extra_data_compatible 780 This flags is meaningful when the extra data is 781 unknown to the software (currently any extra data is 782 unknown to QEMU). 783 If it is set, the bitmap may be used as expected, extra 784 data must be left as is. 785 If it is not set, the bitmap must not be used, but 786 both it and its extra data be left as is. 787 788 Bits 3 - 31 are reserved and must be 0. 789 790 16: type 791 This field describes the sort of the bitmap. 792 Values: 793 1: Dirty tracking bitmap 794 795 Values 0, 2 - 255 are reserved. 796 797 17: granularity_bits 798 Granularity bits. Valid values: 0 - 63. 799 800 Note: QEMU currently supports only values 9 - 31. 801 802 Granularity is calculated as 803 granularity = 1 << granularity_bits 804 805 A bitmap's granularity is how many bytes of the image 806 accounts for one bit of the bitmap. 807 808 18 - 19: name_size 809 Size of the bitmap name. Must be non-zero. 810 811 Note: QEMU currently doesn't support values greater than 812 1023. 813 814 20 - 23: extra_data_size 815 Size of type-specific extra data. 816 817 For now, as no extra data is defined, extra_data_size is 818 reserved and should be zero. If it is non-zero the 819 behavior is defined by extra_data_compatible flag. 820 821 variable: extra_data 822 Extra data for the bitmap, occupying extra_data_size bytes. 823 Extra data must never contain references to clusters or in 824 some other way allocate additional clusters. 825 826 variable: name 827 The name of the bitmap (not null terminated), occupying 828 name_size bytes. Must be unique among all bitmap names 829 within the bitmaps extension. 830 831 variable: Padding to round up the bitmap directory entry size to the 832 next multiple of 8. All bytes of the padding must be zero. 833 834 835=== Bitmap table === 836 837Each bitmap is stored using a one-level structure (as opposed to two-level 838structures like for refcounts and guest clusters mapping) for the mapping of 839bitmap data to host clusters. This structure is called the bitmap table. 840 841Each bitmap table has a variable size (stored in the bitmap directory entry) 842and may use multiple clusters, however, it must be contiguous in the image 843file. 844 845Structure of a bitmap table entry: 846 847 Bit 0: Reserved and must be zero if bits 9 - 55 are non-zero. 848 If bits 9 - 55 are zero: 849 0: Cluster should be read as all zeros. 850 1: Cluster should be read as all ones. 851 852 1 - 8: Reserved and must be zero. 853 854 9 - 55: Bits 9 - 55 of the host cluster offset. Must be aligned to 855 a cluster boundary. If the offset is 0, the cluster is 856 unallocated; in that case, bit 0 determines how this 857 cluster should be treated during reads. 858 859 56 - 63: Reserved and must be zero. 860 861 862=== Bitmap data === 863 864As noted above, bitmap data is stored in separate clusters, described by the 865bitmap table. Given an offset (in bytes) into the bitmap data, the offset into 866the image file can be obtained as follows: 867 868 image_offset(bitmap_data_offset) = 869 bitmap_table[bitmap_data_offset / cluster_size] + 870 (bitmap_data_offset % cluster_size) 871 872This offset is not defined if bits 9 - 55 of bitmap table entry are zero (see 873above). 874 875Given an offset byte_nr into the virtual disk and the bitmap's granularity, the 876bit offset into the image file to the corresponding bit of the bitmap can be 877calculated like this: 878 879 bit_offset(byte_nr) = 880 image_offset(byte_nr / granularity / 8) * 8 + 881 (byte_nr / granularity) % 8 882 883If the size of the bitmap data is not a multiple of the cluster size then the 884last cluster of the bitmap data contains some unused tail bits. These bits must 885be zero. 886 887 888=== Dirty tracking bitmaps === 889 890Bitmaps with 'type' field equal to one are dirty tracking bitmaps. 891 892When the virtual disk is in use dirty tracking bitmap may be 'enabled' or 893'disabled'. While the bitmap is 'enabled', all writes to the virtual disk 894should be reflected in the bitmap. A set bit in the bitmap means that the 895corresponding range of the virtual disk (see above) was written to while the 896bitmap was 'enabled'. An unset bit means that this range was not written to. 897 898The software doesn't have to sync the bitmap in the image file with its 899representation in RAM after each write or metadata change. Flag 'in_use' 900should be set while the bitmap is not synced. 901 902In the image file the 'enabled' state is reflected by the 'auto' flag. If this 903flag is set, the software must consider the bitmap as 'enabled' and start 904tracking virtual disk changes to this bitmap from the first write to the 905virtual disk. If this flag is not set then the bitmap is disabled. 906