docs/interop/qed_spec.txt

5  +----------+----------+----------+-----+
7  +----------+----------+----------+-----+
9 …re extra information about the image file.  A regular cluster may be a '''data cluster''', an '''L…
11 …ultiple, extra information after the last cluster may not be preserved if data is written.  Legiti…
13 All fields are little-endian.
23      uint64_t features;            /* format feature bits */
24      uint64_t compat_features;     /* compat feature bits */
25      uint64_t autoclear_features;  /* self-resetting feature bits */
40 …known ''features'' bits enabled must not be opened.  File format changes that are not backwards-co…
41 …known ''compat_features'' bits enabled can be opened safely.  The unknown features are simply igno…
42 …bits enable can be opened safely after clearing the unknown bits.  This allows for backwards-compa…
45 …me_size'' describe a string in (byte offset, byte size) form.  It is not NUL-terminated and has no…
47 Feature bits:
52 There are currently no defined ''compat_features'' or ''autoclear_features'' bits.
68                     +----------+
70                     +----------+
71                ,------'  |  '------.
72           +----------+   |    +----------+
74           +----------+        +----------+
75       ,------'  |  '------.
76  +----------+   |    +----------+
77  |   Data   |  ...   |   Data   |
78  +----------+        +----------+
85 L1, L2, and data cluster offsets must be aligned to header.cluster_size.  The following offsets hav…
88 * 0 - unallocated.  The L2 table is not yet allocated.
90 ===Data cluster offsets===
91 * 0 - unallocated.  The data cluster is not yet allocated.
92 * 1 - zero.  The data cluster contents are all zeroes and no cluster is allocated.
94 …-offset information.  The least significant 12 bits of an offset are reserved for this purpose and…
96 ===Unallocated L2 tables and data clusters===
99 …data clusters to be allocated, and a new L2 table if that is also unallocated.  The new data clust…
101 ===Zero data clusters===
102 Zero data clusters are a space-efficient way of storing zeroed regions of the image.
104 Reads to a zero data cluster produce zeroes.  Note that the difference between an unallocated and a…
106 Writes to a zero data cluster cause a new data cluster to be allocated.  The new data cluster is po…
112   <--------> <--------> <--------------->
113  +----------+----------+-----------------+
115  +----------+----------+-----------------+
119  offset_mask = ~(cluster_size - 1) # mask for the image file byte offset