UFS2 Group Descriptor

The UFS2 group descriptor has the same basic concepts as the UFS1 version, but some of its fields are larger. Its location relative to the start of the cylinder group is given in the superblock, and its location does not stagger like the UFS1 group descriptor does. The UFS2 version has the fields given in Table 17.6.

Table 17.6. Data structure for the UFS2 group descriptor.

Byte Range	Description	Essential
03	Unused	No
47	Magic signature (0x090255)	No
811	Unused	No
1215	Group number	No
1619	Unused	No
2023	Number of fragments in group	No
2427	Number of directories	No
2831	Number of free blocks	No
3235	Number of free inodes	No
3639	Number of free fragments (partial blocks)	No
4043	Last block allocated	No
4447	Last fragment allocated	No
4851	Last inode allocated	No
5283	Summary of available fragments	No
8491	Unused	No
9295	Inode bitmap (byte offset)	Yes
9699	Fragment bitmap (byte offset)	Yes
100103	Next available space in descriptor (byte offset)	No
104107	Counts of available clusters (consecutive blocks) (byte offset)	No
108111	Block bitmap (byte offset)	No
112115	Number of blocks in group	No
116119	Number of inodes in group	No
120 123	Last initialized inode	No
124135	Unused	No
136143	Last written time	No
144167	Unused	No
168+	Bitmaps and so on	Yes

Notice that in both versions, the only essential information is the offsets to the inode and fragment bitmaps. The bitmaps are located after byte 168, but still inside the block allocated to the group descriptor.