5.6

Disk Interface and Disk Drive Performance
In addition to selecting the appropriate RAID configuration, consider the disk interface and disk drive
performance. VCM data storage needs are usually low enough relative to commonly available drives that
the smallest drives are sufficient. Fast drives that have fast interfaces are important, along with having an
adequate number of spindles (drives) per RAID to distribute read, write, and seek activity across devices.
Most high-end drives are available in 10,000 RPM or 15,000 RPM spin rates. The faster spinning drives
usually seek faster and can achieve a higher sustained data throughput, because more of the platter
surface area passes under the heads in each second.
Two primary interface technologies are suitable for use in high-throughput RAIDS.
n
Ultra 320 SCSI, or U320 supports up to 320MB/s throughput per channel. The HP SmartArray 6404 can
support multiple U320 channels (four for the SA6404) and on-board, battery-backed-up cache. The
cache provides increased read and write performance, because it allows the controller to batch requests
to the drives.
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Serial Attached SCSI (SAS) uses special 2.5” drives and has a data rate up to 600MB/s for newer
controllers, which is higher than the U320. SAS controllers typically have more ports than the channels
in U320 controllers. Ports and channels are similar, because they provide parallel data paths through the
controller. For example, an HP P600 provides 8 ports and each port is capable of 300MB/s.
When you design RAIDs, regardless of the technology, a consideration is to use multiple channels or ports
for high-throughput logical drives. For example, an 8-drive RAID 1+0 on a single U320 channel provides
320MB/s of sustained throughput, while the same drives in a RAID that has four drives on each channel of
a two-channel U320 controller that is striped within the channels and mirrored between channels, provides
640MB/s sustained throughput and offers additional fault tolerance to controller channel or cable
problems. If each quad of drives is in a different cabinet, this setup provides fault tolerance for cabinet
failures.
An alternative to local storage for VCM is to use SAN storage. A common problem with SANs and older
versions of VCM was that many SANs are designed for file server or mailbox use and are not well-suited
to high-throughput OLTP-type activities. For a SAN to provide good performance with VCM, it must be
properly configured internally, and all devices between the SAN and the VCM Collector must be adequate
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.
vCenter Configuration Manager Installation Guide
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