s5 inodes

The term ``inode'' stands for information node. A list of inodes is called an ``i-list'' and the position of an inode in an i-list is called an ``i-number''.

An inode contains all the information about a file except its name, which is kept in a directory. Inodes are 64 bytes long: on a disk with 512-byte physical blocks there are 8 of them in a physical block. The length of an i-list is not fixed; it depends on the number of inodes specified when the filesystem is created.

What does an s5 inode contain?

An s5 inode contains the following:

• type and mode of the file--the type can be regular, directory, block, character, symbolic link, or FIFO, also known as named pipe; the mode is the set of read-write-execute permissions

• number of hard links to the file

• user ID of the owner of the file

• group ID to which the file belongs

• number of bytes in the file

• array of 13 disk block addresses

• date and time the file was last accessed

• date and time the file was last modified

• date and time the file was created

The s5 disk inode's disk block addresses

The array of 13 disk block addresses is the heart of the inode.

• The first ten (addresses 0-9) are direct addresses; that is, they point directly to the first ten logical storage blocks of the contents of the file.

• If the file is larger than ten logical blocks, the 11th address (10) points to an indirect block, which contains direct addresses instead of file contents.

• The 12th address (11) points to a double indirect block, which contains addresses of indirect blocks.

• The 13th address in the array (12) is the address of a triple indirect block, which contains addresses of double indirect blocks.

The address chain in an s5 filesystem

The following table shows the number of bytes addressable by the different levels of indirection in the inode address array for s5 filesystems. These numbers are calculated using the logical block size of the filesystem and the number of bytes (4) used to hold an address.

	Maximum number of bytes addressable by
Logical Block Size	Direct Blocks	Single Indirect Blocks	Double Indirect Blocks	Triple Indirect Blocks
512 bytes	5120	64KB	8MB	1GB
1024 bytes	10240	256KB	64MB	16GB
2048 bytes	20480	1MB	512MB	256GB

The table shows the number of bytes addressable using the level of indirection in the column header plus all lower levels of addressing. For example, the table values for single indirect blocks also include bytes addressable by direct blocks; and the table values for triple indirect blocks include bytes addressable by direct blocks and single and double indirect blocks.

The theoretical maximum size of an s5 system file is the same as the size of a file addressable with triple indirection (shown in the last column of the table). In practice, however, file size is limited by the size field in the inode. This is a 32-bit field, so file sizes are limited to 2GB.