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Tagged Command Queuing intelligently orders the seeks which improves performance for
random accesses. Controller Write Back and Read Ahead Caching allow files to be read and
written most efficiently for those applications accessing large, sequential data and striping
across multiple disks and RAID controller channels.
Tagged Command Queuing
This option allows the I/O subsystem to queue requests and reorder them into the most
efficient order before retrieving them from the disk.
Write-Back Caching
Write-Back Caching allows disk writes to be cached and written when the RAID controller
or disk drive is ready to write. The RAID controller receives data in cache and signals the
software that it has already written the data to the disk. Write-Back Caching 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).
Controller Read Ahead
Under normal operation, Controller Read Ahead is an option on the I/O subsystem that can
improve performance for sequentially accessed data. Controller Read Ahead prefetches
sequential data, assuming the operating system is likely to ask for the prefetched data.
Randomly accessed data may have reduced performance with the Controller Read Ahead
option.
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
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