write to an image with unknown auto-clear features if it
clears the respective bits from this field first.
- Bits 0-63: Reserved (set to 0)
+ Bit 0: Bitmaps extension bit
+ This bit indicates consistency for the bitmaps
+ extension data.
+
+ It is an error if this bit is set without the
+ bitmaps extension present.
+
+ If the bitmaps extension is present but this
+ bit is unset, the bitmaps extension data must be
+ considered inconsistent.
+
+ Bits 1-63: Reserved (set to 0)
96 - 99: refcount_order
Describes the width of a reference count block entry (width
0x00000000 - End of the header extension area
0xE2792ACA - Backing file format name
0x6803f857 - Feature name table
+ 0x23852875 - Bitmaps extension
other - Unknown header extension, can be safely
ignored
terminated if it has full length)
+== Bitmaps extension ==
+
+The bitmaps extension is an optional header extension. It provides the ability
+to store bitmaps related to a virtual disk. For now, there is only one bitmap
+type: the dirty tracking bitmap, which tracks virtual disk changes from some
+point in time.
+
+The data of the extension should be considered consistent only if the
+corresponding auto-clear feature bit is set, see autoclear_features above.
+
+The fields of the bitmaps extension are:
+
+ Byte 0 - 3: nb_bitmaps
+ The number of bitmaps contained in the image. Must be
+ greater than or equal to 1.
+
+ Note: Qemu currently only supports up to 65535 bitmaps per
+ image.
+
+ 4 - 7: Reserved, must be zero.
+
+ 8 - 15: bitmap_directory_size
+ Size of the bitmap directory in bytes. It is the cumulative
+ size of all (nb_bitmaps) bitmap headers.
+
+ 16 - 23: bitmap_directory_offset
+ Offset into the image file at which the bitmap directory
+ starts. Must be aligned to a cluster boundary.
+
+
== Host cluster management ==
qcow2 manages the allocation of host clusters by maintaining a reference count
63: 0 for a cluster that is unused or requires COW, 1 if its
refcount is exactly one. This information is only accurate
- in L2 tables that are reachable from the the active L1
+ in L2 tables that are reachable from the active L1
table.
Standard Cluster Descriptor:
variable: Padding to round up the snapshot table entry size to the
next multiple of 8.
+
+
+== Bitmaps ==
+
+As mentioned above, the bitmaps extension provides the ability to store bitmaps
+related to a virtual disk. This section describes how these bitmaps are stored.
+
+All stored bitmaps are related to the virtual disk stored in the same image, so
+each bitmap size is equal to the virtual disk size.
+
+Each bit of the bitmap is responsible for strictly defined range of the virtual
+disk. For bit number bit_nr the corresponding range (in bytes) will be:
+
+ [bit_nr * bitmap_granularity .. (bit_nr + 1) * bitmap_granularity - 1]
+
+Granularity is a property of the concrete bitmap, see below.
+
+
+=== Bitmap directory ===
+
+Each bitmap saved in the image is described in a bitmap directory entry. The
+bitmap directory is a contiguous area in the image file, whose starting offset
+and length are given by the header extension fields bitmap_directory_offset and
+bitmap_directory_size. The entries of the bitmap directory have variable
+length, depending on the lengths of the bitmap name and extra data. These
+entries are also called bitmap headers.
+
+Structure of a bitmap directory entry:
+
+ Byte 0 - 7: bitmap_table_offset
+ Offset into the image file at which the bitmap table
+ (described below) for the bitmap starts. Must be aligned to
+ a cluster boundary.
+
+ 8 - 11: bitmap_table_size
+ Number of entries in the bitmap table of the bitmap.
+
+ 12 - 15: flags
+ Bit
+ 0: in_use
+ The bitmap was not saved correctly and may be
+ inconsistent.
+
+ 1: auto
+ The bitmap must reflect all changes of the virtual
+ disk by any application that would write to this qcow2
+ file (including writes, snapshot switching, etc.). The
+ type of this bitmap must be 'dirty tracking bitmap'.
+
+ 2: extra_data_compatible
+ This flags is meaningful when the extra data is
+ unknown to the software (currently any extra data is
+ unknown to Qemu).
+ If it is set, the bitmap may be used as expected, extra
+ data must be left as is.
+ If it is not set, the bitmap must not be used, but
+ both it and its extra data be left as is.
+
+ Bits 3 - 31 are reserved and must be 0.
+
+ 16: type
+ This field describes the sort of the bitmap.
+ Values:
+ 1: Dirty tracking bitmap
+
+ Values 0, 2 - 255 are reserved.
+
+ 17: granularity_bits
+ Granularity bits. Valid values: 0 - 63.
+
+ Note: Qemu currently doesn't support granularity_bits
+ greater than 31.
+
+ Granularity is calculated as
+ granularity = 1 << granularity_bits
+
+ A bitmap's granularity is how many bytes of the image
+ accounts for one bit of the bitmap.
+
+ 18 - 19: name_size
+ Size of the bitmap name. Must be non-zero.
+
+ Note: Qemu currently doesn't support values greater than
+ 1023.
+
+ 20 - 23: extra_data_size
+ Size of type-specific extra data.
+
+ For now, as no extra data is defined, extra_data_size is
+ reserved and should be zero. If it is non-zero the
+ behavior is defined by extra_data_compatible flag.
+
+ variable: extra_data
+ Extra data for the bitmap, occupying extra_data_size bytes.
+ Extra data must never contain references to clusters or in
+ some other way allocate additional clusters.
+
+ variable: name
+ The name of the bitmap (not null terminated), occupying
+ name_size bytes. Must be unique among all bitmap names
+ within the bitmaps extension.
+
+ variable: Padding to round up the bitmap directory entry size to the
+ next multiple of 8. All bytes of the padding must be zero.
+
+
+=== Bitmap table ===
+
+Each bitmap is stored using a one-level structure (as opposed to two-level
+structures like for refcounts and guest clusters mapping) for the mapping of
+bitmap data to host clusters. This structure is called the bitmap table.
+
+Each bitmap table has a variable size (stored in the bitmap directory entry)
+and may use multiple clusters, however, it must be contiguous in the image
+file.
+
+Structure of a bitmap table entry:
+
+ Bit 0: Reserved and must be zero if bits 9 - 55 are non-zero.
+ If bits 9 - 55 are zero:
+ 0: Cluster should be read as all zeros.
+ 1: Cluster should be read as all ones.
+
+ 1 - 8: Reserved and must be zero.
+
+ 9 - 55: Bits 9 - 55 of the host cluster offset. Must be aligned to
+ a cluster boundary. If the offset is 0, the cluster is
+ unallocated; in that case, bit 0 determines how this
+ cluster should be treated during reads.
+
+ 56 - 63: Reserved and must be zero.
+
+
+=== Bitmap data ===
+
+As noted above, bitmap data is stored in separate clusters, described by the
+bitmap table. Given an offset (in bytes) into the bitmap data, the offset into
+the image file can be obtained as follows:
+
+ image_offset(bitmap_data_offset) =
+ bitmap_table[bitmap_data_offset / cluster_size] +
+ (bitmap_data_offset % cluster_size)
+
+This offset is not defined if bits 9 - 55 of bitmap table entry are zero (see
+above).
+
+Given an offset byte_nr into the virtual disk and the bitmap's granularity, the
+bit offset into the image file to the corresponding bit of the bitmap can be
+calculated like this:
+
+ bit_offset(byte_nr) =
+ image_offset(byte_nr / granularity / 8) * 8 +
+ (byte_nr / granularity) % 8
+
+If the size of the bitmap data is not a multiple of the cluster size then the
+last cluster of the bitmap data contains some unused tail bits. These bits must
+be zero.
+
+
+=== Dirty tracking bitmaps ===
+
+Bitmaps with 'type' field equal to one are dirty tracking bitmaps.
+
+When the virtual disk is in use dirty tracking bitmap may be 'enabled' or
+'disabled'. While the bitmap is 'enabled', all writes to the virtual disk
+should be reflected in the bitmap. A set bit in the bitmap means that the
+corresponding range of the virtual disk (see above) was written to while the
+bitmap was 'enabled'. An unset bit means that this range was not written to.
+
+The software doesn't have to sync the bitmap in the image file with its
+representation in RAM after each write. Flag 'in_use' should be set while the
+bitmap is not synced.
+
+In the image file the 'enabled' state is reflected by the 'auto' flag. If this
+flag is set, the software must consider the bitmap as 'enabled' and start
+tracking virtual disk changes to this bitmap from the first write to the
+virtual disk. If this flag is not set then the bitmap is disabled.
Code Review / kvmfornfv.git / blobdiff