Managing filesystem types

The sfs inodes

An alternate inode scheme is used for sfs, in which two inodes are allocated for each file:

The first inode, which is always an even-numbered inode, contains common file information and is essentially identical to a ufs inode.
The second (alternate) inode, which is always an odd-numbered inode, contains security information.

Programs that report inode usage show that only even-numbered inodes are in use, because directory entries contain only even inode numbers.

All alternate inodes on the filesystem are preallocated by the mkfs(1M) command.