Veritas File System 4.1 Administrator's Guide (HP-UX 11i v3, February 2007)

Quick I/O for Databases
Quick I/O Functionality and Performance
Chapter 11146
Avoiding Kernel Write Locks
When database I/O is performed via the write() system call, each system call acquires and releases a write
lock inside the kernel. This lock prevents simultaneous write operations on the same file. Because database
systems usually implement their own locks for managing concurrent access to files, write locks
unnecessarily serialize I/O operations. Quick I/O bypasses file system locking and lets the database server
control data access.
Avoiding Double Buffering
Most database servers implement their own buffer cache and do not need the system buffer cache. So the
memory used by the system buffer cache is wasted, and results in data being cached twice: first in the
database cache and then in the system buffer cache. By using direct I/O, Quick I/O does not waste memory
on double buffering. This frees up memory, which the database server buffer cache can use, and thereby
increasing performance.