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Write-Back Policy
The Write-Back option under write policy allows disk writes to be cached and written when
the RAID controller or disk drive is ready to write. The MegaRAID controller receives data
in cache and signals the software that it has already written the data to the disk. Write-Back
boosts performance, but a danger exists that some data could be lost if the power supply is
disrupted during writing. Always protect the data by using a battery backup on the RAID
controller or an uninterruptible power supply (UPS).
Read Ahead Policy
Under normal operation, the Read Ahead option under read policy can improve performance
for sequentially accessed data. The Read Ahead option prefetches sequential data, assuming
that the operating system asks for the prefetched data. Reduced performance can occur for
randomly accessed data.
Database Applications
Place frequently accessed data at the beginning of the disks where the I/O rate generally will
be faster and require less seeking time. Also, place I/O-intensive tables at the beginning of
the disk. Use tablespaces in Oracle, or segments and devices in Microsoft SQL Server and
Sybase Systems to segregate the data for placement on the disk.
To estimate the amount of I/O for tables in the database, use the following formula where
A = table record length, B = number of rows returned in a query, and C = number of times
required to access the table:
A x B x C
For maximum performance in a database, Log or Redo Log access is sequential I/O data; you
should separate sequential I/O data from random I/O data.
Data access is usually random for On-line Transaction Processing (OLTP) applications. The
optimal performance strategy is to stripe the data across devices to distribute the I/O data and
minimize I/O contention. OLTP applications perform operations on small blocks of data
randomly spread across multiple data pages.
Decision Support Systems (DSS) and Data Warehouse applications may benefit from large
block I/O if they are performing large sequential reads of adjacent data pages. Refer to the
documentation for your database system to determine if you can modify the I/O block size.
