be written to (unless for regaining
consistency).
- Bits 2-63: Reserved (set to 0)
+ Bit 2: External data file bit. If this bit is set, an
+ external data file is used. Guest clusters are
+ then stored in the external data file. For such
+ images, clusters in the external data file are
+ not refcounted. The offset field in the
+ Standard Cluster Descriptor must match the
+ guest offset and neither compressed clusters
+ nor internal snapshots are supported.
+
+ An External Data File Name header extension may
+ be present if this bit is set.
+
+ Bits 3-63: Reserved (set to 0)
80 - 87: compatible_features
Bitmask of compatible features. An implementation can
bit is unset, the bitmaps extension data must be
considered inconsistent.
- Bits 1-63: Reserved (set to 0)
+ Bit 1: If this bit is set, the external data file can
+ be read as a consistent standalone raw image
+ without looking at the qcow2 metadata.
+
+ Setting this bit has a performance impact for
+ some operations on the image (e.g. writing
+ zeros requires writing to the data file instead
+ of only setting the zero flag in the L2 table
+ entry) and conflicts with backing files.
+
+ This bit may only be set if the External Data
+ File bit (incompatible feature bit 1) is also
+ set.
+
+ Bits 2-63: Reserved (set to 0)
96 - 99: refcount_order
Describes the width of a reference count block entry (width
Byte 0 - 3: Header extension type:
0x00000000 - End of the header extension area
- 0xE2792ACA - Backing file format name
+ 0xE2792ACA - Backing file format name string
0x6803f857 - Feature name table
0x23852875 - Bitmaps extension
0x0537be77 - Full disk encryption header pointer
+ 0x44415441 - External data file name string
other - Unknown header extension, can be safely
ignored
need space for additional data can use a header extension.
+== String header extensions ==
+
+Some header extensions (such as the backing file format name and the external
+data file name) are just a single string. In this case, the header extension
+length is the string length and the string is not '\0' terminated. (The header
+extension padding can make it look like a string is '\0' terminated, but
+neither is padding always necessary nor is there a guarantee that zero bytes
+are used for padding.)
+
+
== Feature name table ==
The feature name table is an optional header extension that contains the name
This information is only accurate in L2 tables
that are reachable from the active L1 table.
+ With external data files, all guest clusters have an
+ implicit refcount of 1 (because of the fixed host = guest
+ mapping for guest cluster offsets), so this bit should be 1
+ for all allocated clusters.
+
Standard Cluster Descriptor:
Bit 0: If set to 1, the cluster reads as all zeros. The host
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
- unallocated.
+ cluster boundary. If the offset is 0 and bit 63 is clear,
+ the cluster is unallocated. The offset may only be 0 with
+ bit 63 set (indicating a host cluster offset of 0) when an
+ external data file is used.
56 - 61: Reserved (set to 0)
Bit
0: in_use
The bitmap was not saved correctly and may be
- inconsistent.
+ inconsistent. Although the bitmap metadata is still
+ well-formed from a qcow2 perspective, the metadata
+ (such as the auto flag or bitmap size) or data
+ contents may be outdated.
1: auto
The bitmap must reflect all changes of the virtual
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.
+representation in RAM after each write or metadata change. 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
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