diff options
Diffstat (limited to 'docs/interop')
| -rw-r--r-- | docs/interop/qcow2.txt | 54 |
1 files changed, 49 insertions, 5 deletions
diff --git a/docs/interop/qcow2.txt b/docs/interop/qcow2.txt index fb5cb47..8c3098d 100644 --- a/docs/interop/qcow2.txt +++ b/docs/interop/qcow2.txt @@ -97,7 +97,19 @@ in the description of a field. 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 @@ -126,7 +138,21 @@ in the description of a field. 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 @@ -144,10 +170,11 @@ 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 + 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 @@ -169,6 +196,16 @@ data of compatible features that it doesn't support. Compatible features that 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 @@ -437,6 +474,11 @@ L2 table entry: 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 @@ -450,8 +492,10 @@ Standard Cluster Descriptor: 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) |