diff options
Diffstat (limited to 'docs/specs/qcow2.txt')
| -rw-r--r-- | docs/specs/qcow2.txt | 122 |
1 files changed, 99 insertions, 23 deletions
diff --git a/docs/specs/qcow2.txt b/docs/specs/qcow2.txt index b6adcaddb7..65e632555f 100644 --- a/docs/specs/qcow2.txt +++ b/docs/specs/qcow2.txt @@ -18,7 +18,7 @@ The first cluster of a qcow2 image contains the file header: QCOW magic string ("QFI\xfb") 4 - 7: version - Version number (only valid value is 2) + Version number (valid values are 2 and 3) 8 - 15: backing_file_offset Offset into the image file at which the backing file name @@ -67,12 +67,45 @@ The first cluster of a qcow2 image contains the file header: Offset into the image file at which the snapshot table starts. Must be aligned to a cluster boundary. +If the version is 3 or higher, the header has the following additional fields. +For version 2, the values are assumed to be zero, unless specified otherwise +in the description of a field. + + 72 - 79: incompatible_features + Bitmask of incompatible features. An implementation must + fail to open an image if an unknown bit is set. + + Bits 0-63: Reserved (set to 0) + + 80 - 87: compatible_features + Bitmask of compatible features. An implementation can + safely ignore any unknown bits that are set. + + Bits 0-63: Reserved (set to 0) + + 88 - 95: autoclear_features + Bitmask of auto-clear features. An implementation may only + 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) + + 96 - 99: refcount_order + Describes the width of a reference count block entry (width + in bits = 1 << refcount_order). For version 2 images, the + order is always assumed to be 4 (i.e. the width is 16 bits). + + 100 - 103: header_length + Length of the header structure in bytes. For version 2 + images, the length is always assumed to be 72 bytes. + Directly after the image header, optional sections called header extensions can be stored. Each extension has a structure like the following: Byte 0 - 3: Header extension type: 0x00000000 - End of the header extension area 0xE2792ACA - Backing file format name + 0x6803f857 - Feature name table other - Unknown header extension, can be safely ignored @@ -83,9 +116,37 @@ be stored. Each extension has a structure like the following: n - m: Padding to round up the header extension size to the next multiple of 8. +Unless stated otherwise, each header extension type shall appear at most once +in the same image. + The remaining space between the end of the header extension area and the end of -the first cluster can be used for other data. Usually, the backing file name is -stored there. +the first cluster can be used for the backing file name. It is not allowed to +store other data here, so that an implementation can safely modify the header +and add extensions without harming data of compatible features that it +doesn't support. Compatible features that need space for additional data can +use a header extension. + + +== Feature name table == + +The feature name table is an optional header extension that contains the name +for features used by the image. It can be used by applications that don't know +the respective feature (e.g. because the feature was introduced only later) to +display a useful error message. + +The number of entries in the feature name table is determined by the length of +the header extension data. Each entry look like this: + + Byte 0: Type of feature (select feature bitmap) + 0: Incompatible feature + 1: Compatible feature + 2: Autoclear feature + + 1: Bit number within the selected feature bitmap (valid + values: 0-63) + + 2 - 47: Feature name (padded with zeros, but not necessarily null + terminated if it has full length) == Host cluster management == @@ -126,9 +187,11 @@ Refcount table entry: been allocated. All refcounts managed by this refcount block are 0. -Refcount block entry: +Refcount block entry (x = refcount_bits - 1): - Bit 0 - 15: Reference count of the cluster + Bit 0 - x: Reference count of the cluster. If refcount_bits implies a + sub-byte width, note that bit 0 means the least significant + bit in this context. == Cluster mapping == @@ -168,9 +231,29 @@ L1 table entry: refcount is exactly one. This information is only accurate in the active L1 table. -L2 table entry (for normal clusters): +L2 table entry: - Bit 0 - 8: Reserved (set to 0) + Bit 0 - 61: Cluster descriptor + + 62: 0 for standard clusters + 1 for compressed clusters + + 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 + table. + +Standard Cluster Descriptor: + + Bit 0: If set to 1, the cluster reads as all zeros. The host + cluster offset can be used to describe a preallocation, + but it won't be used for reading data from this cluster, + nor is data read from the backing file if the cluster is + unallocated. + + With version 2, this is always 0. + + 1 - 8: Reserved (set to 0) 9 - 55: Bits 9-55 of host cluster offset. Must be aligned to a cluster boundary. If the offset is 0, the cluster is @@ -178,30 +261,18 @@ L2 table entry (for normal clusters): 56 - 61: Reserved (set to 0) - 62: 0 (this cluster is not compressed) - 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 - table. - -L2 table entry (for compressed clusters; x = 62 - (cluster_size - 8)): +Compressed Clusters Descriptor (x = 62 - (cluster_size - 8)): Bit 0 - x: Host cluster offset. This is usually _not_ aligned to a cluster boundary! x+1 - 61: Compressed size of the images in sectors of 512 bytes - 62: 1 (this cluster is compressed using zlib) - - 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 - table. - If a cluster is unallocated, read requests shall read the data from the backing -file. If there is no backing file or the backing file is smaller than the image, -they shall read zeros for all parts that are not covered by the backing file. +file (except if bit 0 in the Standard Cluster Descriptor is set). If there is +no backing file or the backing file is smaller than the image, they shall read +zeros for all parts that are not covered by the backing file. == Snapshots == @@ -261,6 +332,11 @@ Snapshot table entry: state is saved. If this field is present, the 32-bit value in bytes 32-35 is ignored. + Byte 48 - 55: Virtual disk size of the snapshot in bytes + + Version 3 images must include extra data at least up to + byte 55. + variable: Unique ID string for the snapshot (not null terminated) variable: Name of the snapshot (not null terminated) |