root / docs / specs / qcow2.txt @ 03feae73
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== General == |
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|
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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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|
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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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|
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All numbers in qcow2 are stored in Big Endian byte order. |
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|
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|
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== Header == |
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|
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The first cluster of a qcow2 image contains the file header: |
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|
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Byte 0 - 3: magic |
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QCOW magic string ("QFI\xfb") |
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|
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4 - 7: version |
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Version number (only valid value is 2) |
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|
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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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|
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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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|
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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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|
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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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|
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24 - 31: size |
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Virtual disk size in bytes |
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|
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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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|
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36 - 39: l1_size |
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Number of entries in the active L1 table |
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|
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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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|
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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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|
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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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|
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60 - 63: nb_snapshots |
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Number of snapshots contained in the image |
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|
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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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|
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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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|
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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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other - Unknown header extension, can be safely |
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ignored |
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|
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4 - 7: Length of the header extension data |
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|
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8 - n: Header extension data |
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|
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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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|
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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 can be used for other data. Usually, the backing file name is |
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stored there. |
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|
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|
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== Host cluster management == |
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|
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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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|
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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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|
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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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|
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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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|
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refcount_block_entries = (cluster_size / sizeof(uint16_t)) |
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|
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refcount_block_index = (offset / cluster_size) % refcount_table_entries |
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refcount_table_index = (offset / cluster_size) / refcount_table_entries |
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|
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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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|
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Refcount table entry: |
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|
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Bit 0 - 8: Reserved (set to 0) |
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|
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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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|
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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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|
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Refcount block entry: |
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|
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Bit 0 - 15: Reference count of the cluster |
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|
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|
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== Cluster mapping == |
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|
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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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|
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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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|
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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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|
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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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|
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56 - 62: Reserved (set to 0) |
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|
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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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|
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L2 table entry (for normal clusters): |
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Bit 0 - 8: Reserved (set to 0) |
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|
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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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|
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62: 0 (this cluster is not compressed) |
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|
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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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|
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L2 table entry (for compressed clusters; x = 62 - (cluster_size - 8)): |
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|
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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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|
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x+1 - 61: Compressed size of the images in sectors of 512 bytes |
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|
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62: 1 (this cluster is compressed using zlib) |
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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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|
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If a cluster is unallocated, read requests shall read the data from the backing |
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file. If there is no backing file or the backing file is smaller than the image, |
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they shall read zeros for all parts that are not covered by the backing file. |
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|
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|
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== Snapshots == |
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|
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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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|
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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 form 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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|
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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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|
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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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|
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Snapshot table entry: |
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|
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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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|
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8 - 11: Number of entries in the L1 table of the snapshots |
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|
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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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|
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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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|
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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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|
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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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|
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variable: Extra data for future extensions. Must be ignored. |
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|
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variable: Unique ID string for the snapshot (not null terminated) |
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|
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variable: Name of the snapshot (not null terminated) |