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qcow2: Extend spec for external data files

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This adds external data file to the qcow2 spec as a new incompatible
feature.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
This commit is contained in:
Kevin Wolf 2019-01-29 15:25:21 +01:00
parent c5e86ebc11
commit 65a3d073e9
1 changed files with 38 additions and 4 deletions
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42
docs/interop/qcow2.txt
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@ -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
@ -148,6 +174,7 @@ be stored. Each extension has a structure like the following:
0x6803f857 - Feature name table
0x23852875 - Bitmaps extension
0x0537be77 - Full disk encryption header pointer
0x44415441 - External data file name
other - Unknown header extension, can be safely
ignored
@ -437,6 +464,11 @@ 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 +482,10 @@ 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)