Array Controllers; I/O Limits - Compaq 112726-001 - ProLiant - 6500 White Paper

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other drives. Log drives should be placed on a RAID-1 drive, so that no log files are lost in the
case of a single disk failure.

Array Controllers

The Compaq SMART Array Controller is preferable to standard SCSI controllers. The following
are benefits achieved when using the Compaq SMART Array Controllers. This information is
specific to the Compaq SMART-2/P and SMART-2DH controllers.
Allows for up to 14 disks to be configured in the system as one or more logical drives. When
multiple disks are collected into a single array, any logical drives created on that array will
have their data striped across all of the disks. This will more evenly distribute the data
across the disk spindles so that your I/O load will be more balanced on them. This works
really well
for random I/O loads or for sequential loads where you do not have more sequential streams
accessing the data than you have defined logical drives on which the data coexists. For
example, if you spread your table across four logical drives, to maintain sequential streams,
you should keep your degree of parallelism on that table to four or less. Each logical drive
will then be sequentially scanned, which results in a sequential scan of the physical disks.
Multiple logical drives can be configured on each disk or array of disks. The maximum
number of logical drives per single controller is 32. In a DSS system, you will want only one
logical drive per disk.
The accelerator cache can be configured as either read-ahead cache or write-posting cache or
varying degrees of both. The memory for the accelerator is either 4MB or 16MB, depending
upon the version of the controller board. The memory can be configured at 100/0 percent,
75/25 percent, 50/50 percent, 25/75 percent or 0/100 percent for the amounts of memory that
will be reserved for the read-ahead/write-posting functionality of the accelerator. The
configuration you choose will primarily depend upon the amount of writes to your database.
Controller provided RAID levels – Defining RAID levels at the controller versus software
RAID on Windows NT relieves system processor overhead for maintaining RAID.

I/O Limits

In a DSS system, it is optimal to tune for most data access to be sequential for better data
throughput. When multiple processes are accessing a disk concurrently, it is important not to
overload each individual disk with random I/O's. The following are recommended I/O limits for
Compaq 4.3-Gigabyte Ultra and 1-inch and 10K RPM 9.1-Gigabyte drives. For 16K block size,
Compaq recommends that random I/O's not exceed 60 I/O's per second per drive for the 4.3-
Gigabyte Ultra drives, and 75 I/O's per second per drive for 9.1-Gigabyte drives.
While the system is under load, determine the number of I/O's per second to each logical volume
using the Performance Monitor. See Figure 10. Then, using the formulas in the Compaq
Database Engineering White Paper Configuring Compaq RAID Technology for Database
Servers, you can calculate the number of I/O's per physical disk based on the fault tolerance level
of that particular logical volume.
In the example displayed in Figure 10, there is no fault tolerance on logical NTFS volume G.
The average I/O rate is 478.262 I/Os per second. If the drive set consists of seven 4.3-Gigabyte
Ultra drives, then the I/O rate per second per drive is 478.262 / 7 = 68.32 I/O's per second per
drive. The test purposely flooded the logical drive with I/O's, giving a scenario above the
recommended 60 I/O's per second per disk and requiring the possibility of adding more disk
drives or rearranging files. If the drive set consists of seven 9.1-Gigabyte drives, then the 68.32
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