3 A qcow2 image file is organized in units of constant size, which are called
4 (host) clusters. A cluster is the unit in which all allocations are done,
5 both for actual guest data and for image metadata.
7 Likewise, the virtual disk as seen by the guest is divided into (guest)
8 clusters of the same size.
10 All numbers in qcow2 are stored in Big Endian byte order.
15 The first cluster of a qcow2 image contains the file header:
18 QCOW magic string ("QFI\xfb")
21 Version number (valid values are 2 and 3)
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.
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
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).
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.
43 Virtual disk size in bytes.
45 Note: qemu has an implementation limit of 32 MB as
46 the maximum L1 table size. With a 2 MB cluster
47 size, it is unable to populate a virtual cluster
48 beyond 2 EB (61 bits); with a 512 byte cluster
49 size, it is unable to populate a virtual size
50 larger than 128 GB (37 bits). Meanwhile, L1/L2
51 table layouts limit an image to no more than 64 PB
52 (56 bits) of populated clusters, and an image may
53 hit other limits first (such as a file system's
62 Number of entries in the active L1 table
64 40 - 47: l1_table_offset
65 Offset into the image file at which the active L1 table
66 starts. Must be aligned to a cluster boundary.
68 48 - 55: refcount_table_offset
69 Offset into the image file at which the refcount table
70 starts. Must be aligned to a cluster boundary.
72 56 - 59: refcount_table_clusters
73 Number of clusters that the refcount table occupies
76 Number of snapshots contained in the image
78 64 - 71: snapshots_offset
79 Offset into the image file at which the snapshot table
80 starts. Must be aligned to a cluster boundary.
82 For version 2, the header is exactly 72 bytes in length, and finishes here.
83 For version 3 or higher, the header length is at least 104 bytes, including
84 the next fields through header_length.
86 72 - 79: incompatible_features
87 Bitmask of incompatible features. An implementation must
88 fail to open an image if an unknown bit is set.
90 Bit 0: Dirty bit. If this bit is set then refcounts
91 may be inconsistent, make sure to scan L1/L2
92 tables to repair refcounts before accessing the
95 Bit 1: Corrupt bit. If this bit is set then any data
96 structure may be corrupt and the image must not
97 be written to (unless for regaining
100 Bit 2: External data file bit. If this bit is set, an
101 external data file is used. Guest clusters are
102 then stored in the external data file. For such
103 images, clusters in the external data file are
104 not refcounted. The offset field in the
105 Standard Cluster Descriptor must match the
106 guest offset and neither compressed clusters
107 nor internal snapshots are supported.
109 An External Data File Name header extension may
110 be present if this bit is set.
112 Bit 3: Compression type bit. If this bit is set,
113 a non-default compression is used for compressed
114 clusters. The compression_type field must be
115 present and not zero.
117 Bits 4-63: Reserved (set to 0)
119 80 - 87: compatible_features
120 Bitmask of compatible features. An implementation can
121 safely ignore any unknown bits that are set.
123 Bit 0: Lazy refcounts bit. If this bit is set then
124 lazy refcount updates can be used. This means
125 marking the image file dirty and postponing
126 refcount metadata updates.
128 Bits 1-63: Reserved (set to 0)
130 88 - 95: autoclear_features
131 Bitmask of auto-clear features. An implementation may only
132 write to an image with unknown auto-clear features if it
133 clears the respective bits from this field first.
135 Bit 0: Bitmaps extension bit
136 This bit indicates consistency for the bitmaps
139 It is an error if this bit is set without the
140 bitmaps extension present.
142 If the bitmaps extension is present but this
143 bit is unset, the bitmaps extension data must be
144 considered inconsistent.
146 Bit 1: If this bit is set, the external data file can
147 be read as a consistent standalone raw image
148 without looking at the qcow2 metadata.
150 Setting this bit has a performance impact for
151 some operations on the image (e.g. writing
152 zeros requires writing to the data file instead
153 of only setting the zero flag in the L2 table
154 entry) and conflicts with backing files.
156 This bit may only be set if the External Data
157 File bit (incompatible feature bit 1) is also
160 Bits 2-63: Reserved (set to 0)
162 96 - 99: refcount_order
163 Describes the width of a reference count block entry (width
164 in bits: refcount_bits = 1 << refcount_order). For version 2
165 images, the order is always assumed to be 4
166 (i.e. refcount_bits = 16).
167 This value may not exceed 6 (i.e. refcount_bits = 64).
169 100 - 103: header_length
170 Length of the header structure in bytes. For version 2
171 images, the length is always assumed to be 72 bytes.
172 For version 3 it's at least 104 bytes and must be a multiple
176 === Additional fields (version 3 and higher) ===
178 In general, these fields are optional and may be safely ignored by the software,
179 as well as filled by zeros (which is equal to field absence), if software needs
180 to set field B, but does not care about field A which precedes B. More
181 formally, additional fields have the following compatibility rules:
183 1. If the value of the additional field must not be ignored for correct
184 handling of the file, it will be accompanied by a corresponding incompatible
187 2. If there are no unrecognized incompatible feature bits set, an unknown
188 additional field may be safely ignored other than preserving its value when
189 rewriting the image header.
191 3. An explicit value of 0 will have the same behavior as when the field is not
192 present*, if not altered by a specific incompatible bit.
194 *. A field is considered not present when header_length is less than or equal
195 to the field's offset. Also, all additional fields are not present for
198 104: compression_type
200 Defines the compression method used for compressed clusters.
201 All compressed clusters in an image use the same compression
204 If the incompatible bit "Compression type" is set: the field
205 must be present and non-zero (which means non-zlib
206 compression type). Otherwise, this field must not be present
207 or must be zero (which means zlib).
209 Available compression type values:
210 0: zlib <https://www.zlib.net/>
213 === Header padding ===
215 @header_length must be a multiple of 8, which means that if the end of the last
216 additional field is not aligned, some padding is needed. This padding must be
217 zeroed, so that if some existing (or future) additional field will fall into
218 the padding, it will be interpreted accordingly to point [3.] of the previous
219 paragraph, i.e. in the same manner as when this field is not present.
222 === Header extensions ===
224 Directly after the image header, optional sections called header extensions can
225 be stored. Each extension has a structure like the following:
227 Byte 0 - 3: Header extension type:
228 0x00000000 - End of the header extension area
229 0xE2792ACA - Backing file format name string
230 0x6803f857 - Feature name table
231 0x23852875 - Bitmaps extension
232 0x0537be77 - Full disk encryption header pointer
233 0x44415441 - External data file name string
234 other - Unknown header extension, can be safely
237 4 - 7: Length of the header extension data
239 8 - n: Header extension data
241 n - m: Padding to round up the header extension size to the next
244 Unless stated otherwise, each header extension type shall appear at most once
247 If the image has a backing file then the backing file name should be stored in
248 the remaining space between the end of the header extension area and the end of
249 the first cluster. It is not allowed to store other data here, so that an
250 implementation can safely modify the header and add extensions without harming
251 data of compatible features that it doesn't support. Compatible features that
252 need space for additional data can use a header extension.
255 == String header extensions ==
257 Some header extensions (such as the backing file format name and the external
258 data file name) are just a single string. In this case, the header extension
259 length is the string length and the string is not '\0' terminated. (The header
260 extension padding can make it look like a string is '\0' terminated, but
261 neither is padding always necessary nor is there a guarantee that zero bytes
262 are used for padding.)
265 == Feature name table ==
267 The feature name table is an optional header extension that contains the name
268 for features used by the image. It can be used by applications that don't know
269 the respective feature (e.g. because the feature was introduced only later) to
270 display a useful error message.
272 The number of entries in the feature name table is determined by the length of
273 the header extension data. Each entry look like this:
275 Byte 0: Type of feature (select feature bitmap)
276 0: Incompatible feature
277 1: Compatible feature
280 1: Bit number within the selected feature bitmap (valid
283 2 - 47: Feature name (padded with zeros, but not necessarily null
284 terminated if it has full length)
287 == Bitmaps extension ==
289 The bitmaps extension is an optional header extension. It provides the ability
290 to store bitmaps related to a virtual disk. For now, there is only one bitmap
291 type: the dirty tracking bitmap, which tracks virtual disk changes from some
294 The data of the extension should be considered consistent only if the
295 corresponding auto-clear feature bit is set, see autoclear_features above.
297 The fields of the bitmaps extension are:
299 Byte 0 - 3: nb_bitmaps
300 The number of bitmaps contained in the image. Must be
301 greater than or equal to 1.
303 Note: Qemu currently only supports up to 65535 bitmaps per
306 4 - 7: Reserved, must be zero.
308 8 - 15: bitmap_directory_size
309 Size of the bitmap directory in bytes. It is the cumulative
310 size of all (nb_bitmaps) bitmap directory entries.
312 16 - 23: bitmap_directory_offset
313 Offset into the image file at which the bitmap directory
314 starts. Must be aligned to a cluster boundary.
316 == Full disk encryption header pointer ==
318 The full disk encryption header must be present if, and only if, the
319 'crypt_method' header requires metadata. Currently this is only true
320 of the 'LUKS' crypt method. The header extension must be absent for
323 This header provides the offset at which the crypt method can store
324 its additional data, as well as the length of such data.
326 Byte 0 - 7: Offset into the image file at which the encryption
327 header starts in bytes. Must be aligned to a cluster
329 Byte 8 - 15: Length of the written encryption header in bytes.
330 Note actual space allocated in the qcow2 file may
331 be larger than this value, since it will be rounded
332 to the nearest multiple of the cluster size. Any
333 unused bytes in the allocated space will be initialized
336 For the LUKS crypt method, the encryption header works as follows.
338 The first 592 bytes of the header clusters will contain the LUKS
339 partition header. This is then followed by the key material data areas.
340 The size of the key material data areas is determined by the number of
341 stripes in the key slot and key size. Refer to the LUKS format
342 specification ('docs/on-disk-format.pdf' in the cryptsetup source
343 package) for details of the LUKS partition header format.
345 In the LUKS partition header, the "payload-offset" field will be
346 calculated as normal for the LUKS spec. ie the size of the LUKS
347 header, plus key material regions, plus padding, relative to the
348 start of the LUKS header. This offset value is not required to be
349 qcow2 cluster aligned. Its value is currently never used in the
350 context of qcow2, since the qcow2 file format itself defines where
351 the real payload offset is, but none the less a valid payload offset
352 should always be present.
354 In the LUKS key slots header, the "key-material-offset" is relative
355 to the start of the LUKS header clusters in the qcow2 container,
356 not the start of the qcow2 file.
358 Logically the layout looks like
360 +-----------------------------+
362 | QCow2 header extension X |
363 | QCow2 header extension FDE |
364 | QCow2 header extension ... |
365 | QCow2 header extension Z |
366 +-----------------------------+
367 | ....other QCow2 tables.... |
370 +-----------------------------+
371 | +-------------------------+ |
372 | | LUKS partition header | |
373 | +-------------------------+ |
374 | | LUKS key material 1 | |
375 | +-------------------------+ |
376 | | LUKS key material 2 | |
377 | +-------------------------+ |
378 | | LUKS key material ... | |
379 | +-------------------------+ |
380 | | LUKS key material 8 | |
381 | +-------------------------+ |
382 +-----------------------------+
383 | QCow2 cluster payload |
388 +-----------------------------+
390 == Data encryption ==
392 When an encryption method is requested in the header, the image payload
393 data must be encrypted/decrypted on every write/read. The image headers
394 and metadata are never encrypted.
396 The algorithms used for encryption vary depending on the method
400 The AES cipher, in CBC mode, with 256 bit keys.
402 Initialization vectors generated using plain64 method, with
403 the virtual disk sector as the input tweak.
405 This format is no longer supported in QEMU system emulators, due
406 to a number of design flaws affecting its security. It is only
407 supported in the command line tools for the sake of back compatibility
412 The algorithms are specified in the LUKS header.
414 Initialization vectors generated using the method specified
415 in the LUKS header, with the physical disk sector as the
418 == Host cluster management ==
420 qcow2 manages the allocation of host clusters by maintaining a reference count
421 for each host cluster. A refcount of 0 means that the cluster is free, 1 means
422 that it is used, and >= 2 means that it is used and any write access must
423 perform a COW (copy on write) operation.
425 The refcounts are managed in a two-level table. The first level is called
426 refcount table and has a variable size (which is stored in the header). The
427 refcount table can cover multiple clusters, however it needs to be contiguous
430 It contains pointers to the second level structures which are called refcount
431 blocks and are exactly one cluster in size.
433 Although a large enough refcount table can reserve clusters past 64 PB
434 (56 bits) (assuming the underlying protocol can even be sized that
435 large), note that some qcow2 metadata such as L1/L2 tables must point
436 to clusters prior to that point.
438 Note: qemu has an implementation limit of 8 MB as the maximum refcount
439 table size. With a 2 MB cluster size and a default refcount_order of
440 4, it is unable to reference host resources beyond 2 EB (61 bits); in
441 the worst case, with a 512 cluster size and refcount_order of 6, it is
442 unable to access beyond 32 GB (35 bits).
444 Given an offset into the image file, the refcount of its cluster can be
447 refcount_block_entries = (cluster_size * 8 / refcount_bits)
449 refcount_block_index = (offset / cluster_size) % refcount_block_entries
450 refcount_table_index = (offset / cluster_size) / refcount_block_entries
452 refcount_block = load_cluster(refcount_table[refcount_table_index]);
453 return refcount_block[refcount_block_index];
455 Refcount table entry:
457 Bit 0 - 8: Reserved (set to 0)
459 9 - 63: Bits 9-63 of the offset into the image file at which the
460 refcount block starts. Must be aligned to a cluster
463 If this is 0, the corresponding refcount block has not yet
464 been allocated. All refcounts managed by this refcount block
467 Refcount block entry (x = refcount_bits - 1):
469 Bit 0 - x: Reference count of the cluster. If refcount_bits implies a
470 sub-byte width, note that bit 0 means the least significant
474 == Cluster mapping ==
476 Just as for refcounts, qcow2 uses a two-level structure for the mapping of
477 guest clusters to host clusters. They are called L1 and L2 table.
479 The L1 table has a variable size (stored in the header) and may use multiple
480 clusters, however it must be contiguous in the image file. L2 tables are
481 exactly one cluster in size.
483 The L1 and L2 tables have implications on the maximum virtual file
484 size; for a given L1 table size, a larger cluster size is required for
485 the guest to have access to more space. Furthermore, a virtual
486 cluster must currently map to a host offset below 64 PB (56 bits)
487 (although this limit could be relaxed by putting reserved bits into
488 use). Additionally, as cluster size increases, the maximum host
489 offset for a compressed cluster is reduced (a 2M cluster size requires
490 compressed clusters to reside below 512 TB (49 bits), and this limit
491 cannot be relaxed without an incompatible layout change).
493 Given an offset into the virtual disk, the offset into the image file can be
496 l2_entries = (cluster_size / sizeof(uint64_t))
498 l2_index = (offset / cluster_size) % l2_entries
499 l1_index = (offset / cluster_size) / l2_entries
501 l2_table = load_cluster(l1_table[l1_index]);
502 cluster_offset = l2_table[l2_index];
504 return cluster_offset + (offset % cluster_size)
508 Bit 0 - 8: Reserved (set to 0)
510 9 - 55: Bits 9-55 of the offset into the image file at which the L2
511 table starts. Must be aligned to a cluster boundary. If the
512 offset is 0, the L2 table and all clusters described by this
513 L2 table are unallocated.
515 56 - 62: Reserved (set to 0)
517 63: 0 for an L2 table that is unused or requires COW, 1 if its
518 refcount is exactly one. This information is only accurate
519 in the active L1 table.
523 Bit 0 - 61: Cluster descriptor
525 62: 0 for standard clusters
526 1 for compressed clusters
528 63: 0 for clusters that are unused, compressed or require COW.
529 1 for standard clusters whose refcount is exactly one.
530 This information is only accurate in L2 tables
531 that are reachable from the active L1 table.
533 With external data files, all guest clusters have an
534 implicit refcount of 1 (because of the fixed host = guest
535 mapping for guest cluster offsets), so this bit should be 1
536 for all allocated clusters.
538 Standard Cluster Descriptor:
540 Bit 0: If set to 1, the cluster reads as all zeros. The host
541 cluster offset can be used to describe a preallocation,
542 but it won't be used for reading data from this cluster,
543 nor is data read from the backing file if the cluster is
546 With version 2, this is always 0.
548 1 - 8: Reserved (set to 0)
550 9 - 55: Bits 9-55 of host cluster offset. Must be aligned to a
551 cluster boundary. If the offset is 0 and bit 63 is clear,
552 the cluster is unallocated. The offset may only be 0 with
553 bit 63 set (indicating a host cluster offset of 0) when an
554 external data file is used.
556 56 - 61: Reserved (set to 0)
559 Compressed Clusters Descriptor (x = 62 - (cluster_bits - 8)):
561 Bit 0 - x-1: Host cluster offset. This is usually _not_ aligned to a
562 cluster or sector boundary! If cluster_bits is
563 small enough that this field includes bits beyond
564 55, those upper bits must be set to 0.
566 x - 61: Number of additional 512-byte sectors used for the
567 compressed data, beyond the sector containing the offset
568 in the previous field. Some of these sectors may reside
569 in the next contiguous host cluster.
571 Note that the compressed data does not necessarily occupy
572 all of the bytes in the final sector; rather, decompression
573 stops when it has produced a cluster of data.
575 Another compressed cluster may map to the tail of the final
576 sector used by this compressed cluster.
578 If a cluster is unallocated, read requests shall read the data from the backing
579 file (except if bit 0 in the Standard Cluster Descriptor is set). If there is
580 no backing file or the backing file is smaller than the image, they shall read
581 zeros for all parts that are not covered by the backing file.
586 qcow2 supports internal snapshots. Their basic principle of operation is to
587 switch the active L1 table, so that a different set of host clusters are
588 exposed to the guest.
590 When creating a snapshot, the L1 table should be copied and the refcount of all
591 L2 tables and clusters reachable from this L1 table must be increased, so that
592 a write causes a COW and isn't visible in other snapshots.
594 When loading a snapshot, bit 63 of all entries in the new active L1 table and
595 all L2 tables referenced by it must be reconstructed from the refcount table
596 as it doesn't need to be accurate in inactive L1 tables.
598 A directory of all snapshots is stored in the snapshot table, a contiguous area
599 in the image file, whose starting offset and length are given by the header
600 fields snapshots_offset and nb_snapshots. The entries of the snapshot table
601 have variable length, depending on the length of ID, name and extra data.
603 Snapshot table entry:
605 Byte 0 - 7: Offset into the image file at which the L1 table for the
606 snapshot starts. Must be aligned to a cluster boundary.
608 8 - 11: Number of entries in the L1 table of the snapshots
610 12 - 13: Length of the unique ID string describing the snapshot
612 14 - 15: Length of the name of the snapshot
614 16 - 19: Time at which the snapshot was taken in seconds since the
617 20 - 23: Subsecond part of the time at which the snapshot was taken
620 24 - 31: Time that the guest was running until the snapshot was
623 32 - 35: Size of the VM state in bytes. 0 if no VM state is saved.
624 If there is VM state, it starts at the first cluster
625 described by first L1 table entry that doesn't describe a
626 regular guest cluster (i.e. VM state is stored like guest
627 disk content, except that it is stored at offsets that are
628 larger than the virtual disk presented to the guest)
630 36 - 39: Size of extra data in the table entry (used for future
631 extensions of the format)
633 variable: Extra data for future extensions. Unknown fields must be
634 ignored. Currently defined are (offset relative to snapshot
637 Byte 40 - 47: Size of the VM state in bytes. 0 if no VM
638 state is saved. If this field is present,
639 the 32-bit value in bytes 32-35 is ignored.
641 Byte 48 - 55: Virtual disk size of the snapshot in bytes
643 Version 3 images must include extra data at least up to
646 variable: Unique ID string for the snapshot (not null terminated)
648 variable: Name of the snapshot (not null terminated)
650 variable: Padding to round up the snapshot table entry size to the
656 As mentioned above, the bitmaps extension provides the ability to store bitmaps
657 related to a virtual disk. This section describes how these bitmaps are stored.
659 All stored bitmaps are related to the virtual disk stored in the same image, so
660 each bitmap size is equal to the virtual disk size.
662 Each bit of the bitmap is responsible for strictly defined range of the virtual
663 disk. For bit number bit_nr the corresponding range (in bytes) will be:
665 [bit_nr * bitmap_granularity .. (bit_nr + 1) * bitmap_granularity - 1]
667 Granularity is a property of the concrete bitmap, see below.
670 === Bitmap directory ===
672 Each bitmap saved in the image is described in a bitmap directory entry. The
673 bitmap directory is a contiguous area in the image file, whose starting offset
674 and length are given by the header extension fields bitmap_directory_offset and
675 bitmap_directory_size. The entries of the bitmap directory have variable
676 length, depending on the lengths of the bitmap name and extra data.
678 Structure of a bitmap directory entry:
680 Byte 0 - 7: bitmap_table_offset
681 Offset into the image file at which the bitmap table
682 (described below) for the bitmap starts. Must be aligned to
685 8 - 11: bitmap_table_size
686 Number of entries in the bitmap table of the bitmap.
691 The bitmap was not saved correctly and may be
692 inconsistent. Although the bitmap metadata is still
693 well-formed from a qcow2 perspective, the metadata
694 (such as the auto flag or bitmap size) or data
695 contents may be outdated.
698 The bitmap must reflect all changes of the virtual
699 disk by any application that would write to this qcow2
700 file (including writes, snapshot switching, etc.). The
701 type of this bitmap must be 'dirty tracking bitmap'.
703 2: extra_data_compatible
704 This flags is meaningful when the extra data is
705 unknown to the software (currently any extra data is
707 If it is set, the bitmap may be used as expected, extra
708 data must be left as is.
709 If it is not set, the bitmap must not be used, but
710 both it and its extra data be left as is.
712 Bits 3 - 31 are reserved and must be 0.
715 This field describes the sort of the bitmap.
717 1: Dirty tracking bitmap
719 Values 0, 2 - 255 are reserved.
722 Granularity bits. Valid values: 0 - 63.
724 Note: Qemu currently supports only values 9 - 31.
726 Granularity is calculated as
727 granularity = 1 << granularity_bits
729 A bitmap's granularity is how many bytes of the image
730 accounts for one bit of the bitmap.
733 Size of the bitmap name. Must be non-zero.
735 Note: Qemu currently doesn't support values greater than
738 20 - 23: extra_data_size
739 Size of type-specific extra data.
741 For now, as no extra data is defined, extra_data_size is
742 reserved and should be zero. If it is non-zero the
743 behavior is defined by extra_data_compatible flag.
746 Extra data for the bitmap, occupying extra_data_size bytes.
747 Extra data must never contain references to clusters or in
748 some other way allocate additional clusters.
751 The name of the bitmap (not null terminated), occupying
752 name_size bytes. Must be unique among all bitmap names
753 within the bitmaps extension.
755 variable: Padding to round up the bitmap directory entry size to the
756 next multiple of 8. All bytes of the padding must be zero.
761 Each bitmap is stored using a one-level structure (as opposed to two-level
762 structures like for refcounts and guest clusters mapping) for the mapping of
763 bitmap data to host clusters. This structure is called the bitmap table.
765 Each bitmap table has a variable size (stored in the bitmap directory entry)
766 and may use multiple clusters, however, it must be contiguous in the image
769 Structure of a bitmap table entry:
771 Bit 0: Reserved and must be zero if bits 9 - 55 are non-zero.
772 If bits 9 - 55 are zero:
773 0: Cluster should be read as all zeros.
774 1: Cluster should be read as all ones.
776 1 - 8: Reserved and must be zero.
778 9 - 55: Bits 9 - 55 of the host cluster offset. Must be aligned to
779 a cluster boundary. If the offset is 0, the cluster is
780 unallocated; in that case, bit 0 determines how this
781 cluster should be treated during reads.
783 56 - 63: Reserved and must be zero.
788 As noted above, bitmap data is stored in separate clusters, described by the
789 bitmap table. Given an offset (in bytes) into the bitmap data, the offset into
790 the image file can be obtained as follows:
792 image_offset(bitmap_data_offset) =
793 bitmap_table[bitmap_data_offset / cluster_size] +
794 (bitmap_data_offset % cluster_size)
796 This offset is not defined if bits 9 - 55 of bitmap table entry are zero (see
799 Given an offset byte_nr into the virtual disk and the bitmap's granularity, the
800 bit offset into the image file to the corresponding bit of the bitmap can be
801 calculated like this:
803 bit_offset(byte_nr) =
804 image_offset(byte_nr / granularity / 8) * 8 +
805 (byte_nr / granularity) % 8
807 If the size of the bitmap data is not a multiple of the cluster size then the
808 last cluster of the bitmap data contains some unused tail bits. These bits must
812 === Dirty tracking bitmaps ===
814 Bitmaps with 'type' field equal to one are dirty tracking bitmaps.
816 When the virtual disk is in use dirty tracking bitmap may be 'enabled' or
817 'disabled'. While the bitmap is 'enabled', all writes to the virtual disk
818 should be reflected in the bitmap. A set bit in the bitmap means that the
819 corresponding range of the virtual disk (see above) was written to while the
820 bitmap was 'enabled'. An unset bit means that this range was not written to.
822 The software doesn't have to sync the bitmap in the image file with its
823 representation in RAM after each write or metadata change. Flag 'in_use'
824 should be set while the bitmap is not synced.
826 In the image file the 'enabled' state is reflected by the 'auto' flag. If this
827 flag is set, the software must consider the bitmap as 'enabled' and start
828 tracking virtual disk changes to this bitmap from the first write to the
829 virtual disk. If this flag is not set then the bitmap is disabled.