VERITAS Storage Foundation 4.1 Oracle Administrator's Guide

VERITAS File System Prerelease 8 September 2005, 8:54am
36 VERITAS Storage Foundation for Oracle Administrator’s Guide
Extent-Based Allocation
The HFS file system supplied with HP-UX uses block-based allocation schemes that provide good
random access to files and acceptable latency on small files. For larger files, like database files, this
block-based architecture limits throughput. This limitation makes the HFS file system a less than
optimal choice for database environments.
The VxFS file system addresses this performance issue by allocating storage in groups of extents
rather than a block at a time. An extent is one or more adjacent blocks of data within the file system.
An extent is presented as an address-length pair that identifies the starting block address and the
length of the extent (in file system or logical blocks). When storage is allocated to a file on a VxFS
file system, it is grouped in extents, as opposed to being allocated a block at a time as with the HFS
file system.
By allocating disk space to files in extents, disk I/O to and from a file can be done in units of
multiple blocks. This type of I/O can occur if storage is allocated in units of consecutive blocks. For
sequential I/O, multiple block operations are considerably faster than block-at-a-time operations.
Almost all disk drives accept I/O operations of multiple blocks.
The VxFS file system allocates disk space to files in groups of one or more extents. VxFS also
allows applications to control some aspects of the extent allocation for a given file. Extent
attributes are the extent allocation policies associated with a file.
For information on how to create preallocated database files using extent attributes, see
“Preallocating Space for Quick I/O Files Using the setext Command” on page 90.
Fast File System and Database Recovery
VERITAS File System begins recovery procedures within seconds after a system failure by using a
tracking feature called intent logging. This feature records pending changes to the file system
structure in a circular intent log. The intent log recovery feature is not readily apparent to users or a
system administrator except during a system failure. During system failure recovery, the VxFS
fsck utility performs an intent log replay, which scans the intent log and nullifies or completes file
system operations that were active when the system failed. The file system can then be mounted
without completing a full structural check of the entire file system. Replaying the intent log may
not completely recover the damaged file system structure if there was a disk hardware failure;
hardware problems may require a complete system check using the fsck utility provided with
VERITAS File System.
Online System Administration
The VxFS file system provides online system administration utilities to help resolve certain
problems that impact database performance. You can defragment and resize a VxFS file system
while it remains online and accessible to users.