HP JFS 3.3 and HP OnLineJFS 3.3 VERITAS File System 3.3 System Administrator's Guide

Chapter 2 49
Disk Layout
The VxFS Version 1 Disk Layout
Allocation Unit Summary
The allocation unit summary contains the number of inodes with
extended operations pending, the number of free inodes, and the number
of free extents in the allocation unit.
Free Inode Map The free inode map is a bitmap that indicates which
inodes are free and which are allocated. A free inode is indicated by the
bit being on. Inodes zero and one are reserved by the file system; inode
two is the inode for the root directory; inode three is the inode for the
lost+found directory.
Extended Inode Operations Map The extended inode operations
map keeps track of inodes on which operations would remain pending for
too long to reside in the intent log. The extended inode operations map is
in the same format as the free inode map. To prevent the intent log from
wrapping and the transaction from getting overwritten, the required
operations are stored in the affected inode (if the transaction has not
completed, it does not get overwritten, the new log waits and the file
system is frozen). This map is then updated to identify the inodes that
have extended operations that need to be completed.
Free Extent Map The free extent map is a series of independent
512-byte bitmaps that are each referred to as a free extent map section.
Each section is broken down into multiple regions. The first region, of
2048 bits, represents a section of 2048 one-block extents. The second
region, of 1024 bits, represents a section of 1024 two-block extents. This
regioning continues for all powers of 2 up to the single bit that represents
one 2048 block extent.
The file system uses this bitmapping scheme to find an available extent
closest in size to the space required. This keeps files as contiguous as
possible for faster performance.
Inode List An inode is a data structure that contains information about
a file. The VxFS inode size is 256 bytes. Each inode stores information
about a particular file such as:
• file length
• link count
• owner and group IDs
• access privileges