Separate Sequential And Random I/O's; Layout Of Tables And Files - Compaq 108164-003 - ProLiant - 800 White Paper

Configuration and Tuning of Sybase System 11 for NetWare on Compaq Servers
Sequential I/O's are isolated to a controller volume, separate from volumes with random
I/O's. This means sequential I/O volume should be alone on a controller or on one port of
the controller.
Random I/O's are balanced across all drives allocated to data and indexes.
Physical disk I/O limits are not exceeded.

Separate Sequential and Random I/O's

In order to achieve maximum performance on data files being accessed sequentially, the disk(s)
need to be dedicated to this purpose. Of primary importance are the Sybase transaction log files,
which are accessed in a sequential, write-only fashion. Other partitions with little I/O activity
can share the disk(s) with the transaction logs, such as the OS partition.
In typical multi-user database systems, data access is random. This data should be spread out
over as many physical disks as necessary to achieve random I/O rates that do not exceed
recommendations. This is best achieved by using the disk striping available with the Compaq
SMART SCSI Array Controller and SMART-2 SCSI Array Controller. Spreading out the disk
requests among many disks allows a high degree of parallelism to occur on accesses. Using the
Compaq SMART or SMART-2 SCSI Array Controller ensures that the load will be balanced
equally across the disks. For more information on optimizing array configurations refer to the
Compaq TechNote, Configuring Compaq RAID Technology for Database Servers.

Layout of Tables and Files

In order to improve performance where disk I/O is a problem, keep in mind the following.
Transaction log access is 100% sequential I/O and needs to be isolated if possible. Speed of
the log is essential to the performance of the system. If possible, these drives should be fault
tolerant, either mirrored or distributed data guarding. Hardware fault tolerance provides the
maximum performance and reliability. See the Compaq TechNote Configuring Compaq
RAID Technology for Database Servers.
Data file access is usually random and needs to be spread across as many drives as necessary.
By increasing the number of physical drives, greater I/O rates can be achieved. Using a
striped array will assure that the I/O's are well distributed.
Use the following guidelines when monitoring and optimizing the drive subsystem. You should
not have more I/O requests (disk transfers) per second per disk drive than the values in the
following table.
Sequential Writes
(Transaction Log)
Random Reads/Writes
(Database Access)
NOTE: With the Array Accelerator enabled, you may actually see substantially higher I/O per
second per drive rates than suggested above. This increase is due to the Array Accelerator write
posting some of these I/Os. In the Compaq Database Performance labs we have actually
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1.0GB drives 2.1GB drives
(Max I/Os per (Max I/Os per
Second per Drive) Second per Drive)
150
30-40
Page 13
4.3GB drives
(Max I/Os per
Second per Drive)
160 180
50 55-60
Doc No 140A/0896