System Administration Guide: Devices and File Systems

Structure of Cylinder Groups for UFS File Systems

When you create a UFS file system, the disk slice is divided into cylinder groups. A cylinder group is comprised of one or more consecutive disk cylinders. Cylinder groups are then further divided into addressable blocks to control and organize the structure of the files within the cylinder group. Each type of block has a specific function in the file system. A UFS file system has these four types of blocks.

Block Type  

Type of Information Stored 

Boot block 

Information used when the system is booted 


Detailed information about the file system 


All information about a file 

Storage or data block 

Data for each file 

The following sections provide additional information about the organization and function of these blocks.

Boot Block

The boot block stores objects that are used in booting the system. If a file system is not to be used for booting, the boot block is left blank. The boot block appears only in the first cylinder group (cylinder group 0) and is the first 8 KB in a slice.


The superblock stores much of the information about the file system, which includes the following:

Because the superblock contains critical data, multiple superblocks are made when the file system is created.

A summary information block is kept within the superblock. The summary information block is not replicated, but is grouped with the primary superblock, usually in cylinder group 0. The summary block records changes that take place as the file system is used. In addition, the summary block lists the number of inodes, directories, fragments, and storage blocks within the file system.


An inode contains all the information about a file except its name, which is kept in a directory. An inode is 128 bytes. The inode information is kept in the cylinder information block, and contains the following:

The array of 15 disk-block addresses (0 to 14) points to the data blocks that store the contents of the file. The first 12 are direct addresses. That is, they point directly to the first 12 logical storage blocks of the file contents. If the file is larger than 12 logical blocks, the 13th address points to an indirect block, which contains direct-block addresses instead of file contents. The 14th address points to a double indirect block, which contains addresses of indirect blocks. The 15th address is for triple indirect addresses. The following figure shows this chaining of address blocks starting from the inode.

Figure 22–1 Address Chain for a UFS File System

Graphic of relationship between the address array of
a UFS inode and the indirect and double indirect pointers to a file's storage

Data Blocks

Data blocks, also called storage blocks, contain the rest of the space that is allocated to the file system. The size of these data blocks is determined when a file system is created. By default, data blocks are allocated in two sizes: an 8-KB logical block size, and a 1-KB fragment size.

For a regular file, the data blocks contain the contents of the file. For a directory, the data blocks contain entries that give the inode number and the file name of the files in the directory.

Free Blocks

Blocks that are not currently being used as inodes, as indirect address blocks, or as storage blocks are marked as free in the cylinder group map. This map also keeps track of fragments to prevent fragmentation from degrading disk performance.

To give you an idea of the structure of a typical UFS file system, the following figure shows a series of cylinder groups in a generic UFS file system.

Figure 22–2 A Typical UFS File System

Graphic of UFS cylinder groups with boot blocks (8 KB
in cylinder group 0 only), superblock, cylinder group map, inodes, and storage