Common Misconfigured HP-UX Resources (April 2006)

The JFS Metadata Buffer Cache
In past releases of the VERITAS File System (HP OnlineJFS/JFS) prior to Journaled File System (JFS)
3.5, the metadata of a file system was cached in the standard HP-UX buffer cache with all of the user
file data. Beginning with JFS 3.5 introduced on HP-UX 11i v1, the JFS metadata was moved to a
special buffer cache known as the JFS metadata buffer cache (or metadata cache). This cache is
managed separately from the HP-UX buffer cache. This metadata cache serves the same purpose as
the HP-UX buffer cache, but enhancements were made to increase performance due to the unique
ways the metadata is accessed.
This section will address the following questions regarding the metadata cache:
• What is metadata?
• Is the metadata cache static or dynamic?
• How much memory is required for the metadata cache?
• How can the metadata cache be tuned?
• Are there any guidelines for configuring the metadata cache?
What is Metadata?
Metadata is structural information from disk such as inodes, indirect block maps, bitmaps, and
summaries.
If you consider an actual file on disk, it is made up of the inode and data blocks, and potentially
indirect blocks. The inode contains an extent map to either the data blocks or other extent maps
known as indirect blocks.
When inodes are first read in from disk, the file system reads in an entire block of inodes from disk
into the metadata cache (similar to reading a data file). Then, the inodes that are actually being
accessed will be brought into the JFS inode cache. Note the difference between inodes in the
metadata cache, which contains only the disk copy, and the inode in the JFS inode cache, which
contains the linked lists for hashing and free lists, the vnode, locking structures, and the on-disk copy
of the inode.
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