363 lines
15 KiB
Plaintext
363 lines
15 KiB
Plaintext
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== General ==
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A qcow2 image file is organized in units of constant size, which are called
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(host) clusters. A cluster is the unit in which all allocations are done,
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both for actual guest data and for image metadata.
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Likewise, the virtual disk as seen by the guest is divided into (guest)
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clusters of the same size.
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All numbers in qcow2 are stored in Big Endian byte order.
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== Header ==
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The first cluster of a qcow2 image contains the file header:
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Byte 0 - 3: magic
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QCOW magic string ("QFI\xfb")
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4 - 7: version
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Version number (valid values are 2 and 3)
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8 - 15: backing_file_offset
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Offset into the image file at which the backing file name
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is stored (NB: The string is not null terminated). 0 if the
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image doesn't have a backing file.
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16 - 19: backing_file_size
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Length of the backing file name in bytes. Must not be
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longer than 1023 bytes. Undefined if the image doesn't have
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a backing file.
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20 - 23: cluster_bits
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Number of bits that are used for addressing an offset
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within a cluster (1 << cluster_bits is the cluster size).
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Must not be less than 9 (i.e. 512 byte clusters).
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Note: qemu as of today has an implementation limit of 2 MB
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as the maximum cluster size and won't be able to open images
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with larger cluster sizes.
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24 - 31: size
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Virtual disk size in bytes
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32 - 35: crypt_method
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0 for no encryption
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1 for AES encryption
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36 - 39: l1_size
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Number of entries in the active L1 table
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40 - 47: l1_table_offset
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Offset into the image file at which the active L1 table
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starts. Must be aligned to a cluster boundary.
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48 - 55: refcount_table_offset
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Offset into the image file at which the refcount table
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starts. Must be aligned to a cluster boundary.
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56 - 59: refcount_table_clusters
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Number of clusters that the refcount table occupies
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60 - 63: nb_snapshots
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Number of snapshots contained in the image
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64 - 71: snapshots_offset
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Offset into the image file at which the snapshot table
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starts. Must be aligned to a cluster boundary.
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If the version is 3 or higher, the header has the following additional fields.
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For version 2, the values are assumed to be zero, unless specified otherwise
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in the description of a field.
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72 - 79: incompatible_features
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Bitmask of incompatible features. An implementation must
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fail to open an image if an unknown bit is set.
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Bit 0: Dirty bit. If this bit is set then refcounts
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may be inconsistent, make sure to scan L1/L2
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tables to repair refcounts before accessing the
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image.
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Bit 1: Corrupt bit. If this bit is set then any data
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structure may be corrupt and the image must not
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be written to (unless for regaining
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consistency).
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Bits 2-63: Reserved (set to 0)
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80 - 87: compatible_features
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Bitmask of compatible features. An implementation can
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safely ignore any unknown bits that are set.
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Bit 0: Lazy refcounts bit. If this bit is set then
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lazy refcount updates can be used. This means
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marking the image file dirty and postponing
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refcount metadata updates.
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Bits 1-63: Reserved (set to 0)
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88 - 95: autoclear_features
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Bitmask of auto-clear features. An implementation may only
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write to an image with unknown auto-clear features if it
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clears the respective bits from this field first.
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Bits 0-63: Reserved (set to 0)
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96 - 99: refcount_order
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Describes the width of a reference count block entry (width
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in bits: refcount_bits = 1 << refcount_order). For version 2
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images, the order is always assumed to be 4
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(i.e. refcount_bits = 16).
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This value may not exceed 6 (i.e. refcount_bits = 64).
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100 - 103: header_length
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Length of the header structure in bytes. For version 2
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images, the length is always assumed to be 72 bytes.
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Directly after the image header, optional sections called header extensions can
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be stored. Each extension has a structure like the following:
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Byte 0 - 3: Header extension type:
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0x00000000 - End of the header extension area
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0xE2792ACA - Backing file format name
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0x6803f857 - Feature name table
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other - Unknown header extension, can be safely
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ignored
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4 - 7: Length of the header extension data
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8 - n: Header extension data
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n - m: Padding to round up the header extension size to the next
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multiple of 8.
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Unless stated otherwise, each header extension type shall appear at most once
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in the same image.
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If the image has a backing file then the backing file name should be stored in
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the remaining space between the end of the header extension area and the end of
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the first cluster. It is not allowed to store other data here, so that an
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implementation can safely modify the header and add extensions without harming
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data of compatible features that it doesn't support. Compatible features that
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need space for additional data can use a header extension.
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== Feature name table ==
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The feature name table is an optional header extension that contains the name
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for features used by the image. It can be used by applications that don't know
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the respective feature (e.g. because the feature was introduced only later) to
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display a useful error message.
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The number of entries in the feature name table is determined by the length of
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the header extension data. Each entry look like this:
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Byte 0: Type of feature (select feature bitmap)
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0: Incompatible feature
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1: Compatible feature
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2: Autoclear feature
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1: Bit number within the selected feature bitmap (valid
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values: 0-63)
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2 - 47: Feature name (padded with zeros, but not necessarily null
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terminated if it has full length)
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== Host cluster management ==
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qcow2 manages the allocation of host clusters by maintaining a reference count
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for each host cluster. A refcount of 0 means that the cluster is free, 1 means
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that it is used, and >= 2 means that it is used and any write access must
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perform a COW (copy on write) operation.
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The refcounts are managed in a two-level table. The first level is called
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refcount table and has a variable size (which is stored in the header). The
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refcount table can cover multiple clusters, however it needs to be contiguous
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in the image file.
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It contains pointers to the second level structures which are called refcount
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blocks and are exactly one cluster in size.
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Given a offset into the image file, the refcount of its cluster can be obtained
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as follows:
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refcount_block_entries = (cluster_size * 8 / refcount_bits)
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refcount_block_index = (offset / cluster_size) % refcount_block_entries
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refcount_table_index = (offset / cluster_size) / refcount_block_entries
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refcount_block = load_cluster(refcount_table[refcount_table_index]);
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return refcount_block[refcount_block_index];
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Refcount table entry:
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Bit 0 - 8: Reserved (set to 0)
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9 - 63: Bits 9-63 of the offset into the image file at which the
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refcount block starts. Must be aligned to a cluster
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boundary.
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If this is 0, the corresponding refcount block has not yet
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been allocated. All refcounts managed by this refcount block
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are 0.
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Refcount block entry (x = refcount_bits - 1):
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Bit 0 - x: Reference count of the cluster. If refcount_bits implies a
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sub-byte width, note that bit 0 means the least significant
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bit in this context.
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== Cluster mapping ==
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Just as for refcounts, qcow2 uses a two-level structure for the mapping of
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guest clusters to host clusters. They are called L1 and L2 table.
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The L1 table has a variable size (stored in the header) and may use multiple
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clusters, however it must be contiguous in the image file. L2 tables are
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exactly one cluster in size.
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Given a offset into the virtual disk, the offset into the image file can be
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obtained as follows:
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l2_entries = (cluster_size / sizeof(uint64_t))
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l2_index = (offset / cluster_size) % l2_entries
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l1_index = (offset / cluster_size) / l2_entries
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l2_table = load_cluster(l1_table[l1_index]);
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cluster_offset = l2_table[l2_index];
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return cluster_offset + (offset % cluster_size)
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L1 table entry:
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Bit 0 - 8: Reserved (set to 0)
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9 - 55: Bits 9-55 of the offset into the image file at which the L2
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table starts. Must be aligned to a cluster boundary. If the
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offset is 0, the L2 table and all clusters described by this
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L2 table are unallocated.
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56 - 62: Reserved (set to 0)
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63: 0 for an L2 table that is unused or requires COW, 1 if its
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refcount is exactly one. This information is only accurate
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in the active L1 table.
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L2 table entry:
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Bit 0 - 61: Cluster descriptor
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62: 0 for standard clusters
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1 for compressed clusters
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63: 0 for a cluster that is unused or requires COW, 1 if its
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refcount is exactly one. This information is only accurate
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in L2 tables that are reachable from the the active L1
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table.
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Standard Cluster Descriptor:
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Bit 0: If set to 1, the cluster reads as all zeros. The host
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cluster offset can be used to describe a preallocation,
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but it won't be used for reading data from this cluster,
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nor is data read from the backing file if the cluster is
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unallocated.
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With version 2, this is always 0.
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1 - 8: Reserved (set to 0)
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9 - 55: Bits 9-55 of host cluster offset. Must be aligned to a
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cluster boundary. If the offset is 0, the cluster is
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unallocated.
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56 - 61: Reserved (set to 0)
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Compressed Clusters Descriptor (x = 62 - (cluster_bits - 8)):
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Bit 0 - x: Host cluster offset. This is usually _not_ aligned to a
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cluster boundary!
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x+1 - 61: Compressed size of the images in sectors of 512 bytes
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If a cluster is unallocated, read requests shall read the data from the backing
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file (except if bit 0 in the Standard Cluster Descriptor is set). If there is
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no backing file or the backing file is smaller than the image, they shall read
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zeros for all parts that are not covered by the backing file.
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== Snapshots ==
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qcow2 supports internal snapshots. Their basic principle of operation is to
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switch the active L1 table, so that a different set of host clusters are
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exposed to the guest.
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When creating a snapshot, the L1 table should be copied and the refcount of all
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L2 tables and clusters reachable from this L1 table must be increased, so that
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a write causes a COW and isn't visible in other snapshots.
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When loading a snapshot, bit 63 of all entries in the new active L1 table and
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all L2 tables referenced by it must be reconstructed from the refcount table
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as it doesn't need to be accurate in inactive L1 tables.
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A directory of all snapshots is stored in the snapshot table, a contiguous area
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in the image file, whose starting offset and length are given by the header
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fields snapshots_offset and nb_snapshots. The entries of the snapshot table
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have variable length, depending on the length of ID, name and extra data.
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Snapshot table entry:
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Byte 0 - 7: Offset into the image file at which the L1 table for the
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snapshot starts. Must be aligned to a cluster boundary.
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8 - 11: Number of entries in the L1 table of the snapshots
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12 - 13: Length of the unique ID string describing the snapshot
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14 - 15: Length of the name of the snapshot
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16 - 19: Time at which the snapshot was taken in seconds since the
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Epoch
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20 - 23: Subsecond part of the time at which the snapshot was taken
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in nanoseconds
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24 - 31: Time that the guest was running until the snapshot was
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taken in nanoseconds
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32 - 35: Size of the VM state in bytes. 0 if no VM state is saved.
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If there is VM state, it starts at the first cluster
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described by first L1 table entry that doesn't describe a
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regular guest cluster (i.e. VM state is stored like guest
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disk content, except that it is stored at offsets that are
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larger than the virtual disk presented to the guest)
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36 - 39: Size of extra data in the table entry (used for future
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extensions of the format)
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variable: Extra data for future extensions. Unknown fields must be
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ignored. Currently defined are (offset relative to snapshot
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table entry):
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Byte 40 - 47: Size of the VM state in bytes. 0 if no VM
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state is saved. If this field is present,
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the 32-bit value in bytes 32-35 is ignored.
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Byte 48 - 55: Virtual disk size of the snapshot in bytes
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Version 3 images must include extra data at least up to
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byte 55.
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variable: Unique ID string for the snapshot (not null terminated)
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variable: Name of the snapshot (not null terminated)
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variable: Padding to round up the snapshot table entry size to the
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next multiple of 8.
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