Fault Tolerance Considerations - Compaq 117755-003 - ProSignia - 740 Configuration Manual

Configuration and Tuning of Sybase System 11 for Microsoft Windows NT on Compaq Servers
There are three main write-intensive operations Sybase SQL Server performs: checkpoints,
housekeeper and transaction log writes.
Y
During checkpoints, Sybase SQL Server generates a large number of write requests in a
short time interval. The main objective of the checkpoint is to write all dirty pages from the
data cache to the disk in the shortest time possible by simultaneously writing as many pages
as possible. This technique requires careful tuning considerations for the Compaq SMART
and SMART-2 SCSI Array Controllers.
In some environments, the amount of write activity that the checkpoint generates can
saturate the Array Accelerator, thus interfering with read requests pending at the controller.
Proper tuning of the housekeeper can help alleviate this problem.
Y
The housekeeper, if enabled, waits until it detects idle time in the Sybase SQL Server then
begins to write dirty pages from the data cache to the disk at a lower priority than the
checkpoint process. The main objective of the housekeeper is to write as many dirty pages
from the data cache to the disk in the shortest time possible with minimal impact on the
throughput of the database application. Unlike a checkpoint process which must write all
dirty pages from the data cache to disk before terminating, the housekeeper writes only what
it can during idle times. If the system is idle for a long enough period of time the
housekeeper may actually have written all dirty pages from the data cache to disk. When
this occurs the housekeeper notifies the checkpoint process and requests that a checkpoint be
performed on the database.
Y
The transaction log activity is composed exclusively of sequential writes and does not
saturate the Array Accelerator. However, the difference between writing to the Array
Accelerator and writing to the disk drives can be substantial. For optimal performance the
Array Accelerator should be enabled.
The Array Accelerator improves performance of all write activity, including the transaction log
writes, the housekeeper writes and the checkpoint writes.

Fault Tolerance Considerations

Your transaction log needs to always be protected against a disk failure. Protecting your database
devices is at your own discretion. Most mission critical sites protect both the transaction log and
the database devices and they usually choose hardware-based fault tolerance. Non-mission
critical sites are often satisfied with the protection of the transaction log only and do frequent
backups/dumps.
You have three choices of protecting your data:
Use a RAID-based disk controller such as the Compaq SMART and SMART-2
SCSI Array Controllers
Use Windows NT-based RAID
Use Sybase SQL Server-based mirroring/duplexing.
Keep in mind that in an OLTP environment the transaction log activity is characterized by
sequential writes, whereas the database activity is composed of random reads and writes. In a
DSS environment the transaction log activity is characterized by sequential writes, whereas the
database activity is composed mostly of sequential read with few random reads and writes.
Below are some key points to be aware of when choosing the appropriate fault tolerant method.
The performance differences between fault tolerance implementations can vary from insignificant
to very significant, depending on your configuration and environment. Therefore, we omitted
supplying performance differences for the purpose of not misleading our readers. We
recommend that you evaluate the various fault tolerance methods using your own applications.
1996 Compaq Computer Corporation, All Rights Reserved
Page 5
Doc No 143A/0596