2.1 Disk Structure and Partitions

2.1.3 Cylinder group

SunOS uses the Berkeley fast file system which uses cylinder groups. A group is formed form 32 or fewer cylinders on a disk (default 16). Each cylinder group has a redundant copy of the superblock, space for inodes, list of available blocks, and a list of data block usage within the cylinder group. Data blocks are spaced to minimize rotational delays and to keep blocks of the same file close together. By grouping cylinders in this way we reduce the amount of head movement, on average, required to access a file. The inode describing the file, and the data for the file, are likely to be in the same physical area of the disk. The position of the redundant superblock within each cylinder group is varied, so that they don't all reside on the same disk platter. This helps to insure that you can recover in the event of the loss of the primary superblock.

Unix System Administration - 8 AUG 1996

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