Installation guide

for the task. A 4Gb HBA is capable of slightly higher throughput than the single Ultra 320 SCSI channel.
For write activities, because mirroring and striping is handled internally at the SAN, the throughput of the
4Gb HBA is more comparable to two and a half U320 channels. Achieving that throughput also depends
on the switches and links between the Collector and the SAN, and between the drives and the controllers
in the SAN.
When considering SAN storage for VCM, consider throughput, which includes the read and write speed,
and access latency. Throughput and latency are important factors, because VCM performs many relatively
small reads and writes. If the latency is too high, performance will be impacted as SQL Server waits for
responses to small queries before it can perform the next task.
After you install a VCM Collector, use Performance Monitor to analyze the performance of the disk
subsystem. The main counters of interest are the Physical Disk object’s Disk Bytes/sec and Average Disk
Queue Length counters. You can monitor both of these counters on a per-instance basis to determine the
throughput and the number of threads that are queued for each logical drive that is associated with VCM
activity.
The Disk Queue Length value is the best initial indicator on whether a logical drive has sufficient
throughput and access speed for the tasks being required. The Disk Queue Length should not typically be
more than twice the number of processors in the system for more than very short periods of time. When
viewing this counter, a logical drive that is also used by the page file might show high queuing due to
insufficient RAM, but the counter can be useful to determine whether disk subsystem resources are
appropriate and whether the resources are optimally arranged, such as disks per channel, RAID type, and
so on.
Use SQLIO to Determine I/O Channel Throughput
SQLIO is a tool that determines the I/O capacity of a SQL configuration. To predict how well VCM will
function on a particular I/O configuration and to obtain a baseline of how well the I/O subsystem
functions, run SQLIO before you install VCM.
After you download and install SQLIO, configure the following SQLIO settings to ensure an accurate
report of I/O throughput.
n 64K Block Size
n 4 Threads
n 2GB File Size minimum
n Sequential I/O
When you execute SQLIO, verify that you create baseline information for each I/O channel (logical disk) to
be used for VCM data, as well as testing both read and write operations.
Configuring SQL Server for VCM
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