ISS Technology Update, Volume 9, Number 6

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long as the server can deliver the next chunk of the video stream in the required window of time. However, if the latency
exceeds 1000 milliseconds, you will probably see a glitch or a pause in an on-demand movie.
Smart Array controller optimizations for VOD
Smart Array engineers analyze VOD requirements and use this information to optimize the Smart Array controller using the
Array Configuration Utility (ACU). Some configuration parameters require a Smart Array Advance Pack (SAAP) license to
access. Table 3-1 summarizes the optimal ACU configuration settings for VOD and their availability.
Table 3-1. Smart Array ACU configuration settings for VOD optimization
Configuration Parameter
Setting
Availability
Physical Drive Request Elevator Sort
OFF
SAAP
Parity RAID Degraded Mode Performance
Optimization
ON
SAAP
Cache Ratio
100% Write
Standard Smart Array
MNP Delay
0 (Disabled)
SAAP
MNP Data Collection
0 (Disabled)
SAAP
Rebuild Priority
Low
Standard Smart Array
Background Surface Scan Interval
30 seconds
Standard Smart Array
Physical Drive Request Elevator Sort (Table 3-1) can increase the performance of a Smart Array. It lets the controller re-order
block requests to the physical drives to minimize the drives’ seek time. Unfortunately, it also increases the maximum latency
since the controller may execute an earlier block request after it executes subsequent requests. You can turn this function OFF
to decrease the variability of latency.
When an array drive fails, the array operates in Degraded Mode until you replace the failed drive and the Smart Array
controller rebuilds it. During this period, the controller performs multiple disk operations to reconstruct degraded data while it
fulfills read requests. This increases latency. Our engineers found that turning RAID Degraded Mode Performance
Optimization to ON (Table 3-1) lets the Smart Array controller make greater use of the controller memory for regeneration
operations, minimizing but not eliminating the impact on latency during array rebuilds.
Setting the Cache Ratio to 100% Write (Table 3-1) may seem odd for VOD. However, engineers discovered that using the
Smart Array controller memory for read cache does not increase read performance for most VOD installations. Read caching
is ineffective due to the amount of large block data the VOD server reads combined with the number of streams it makes.
Setting the Cache Ratio to 100% Write allows the Smart Array controller to post infrequent writes to the cache and return to
fulfilling read requests as quickly as possible.
Engineers minimize the intrusion of Smart Array controller background tasks by disabling Monitor and Performance (MNP);
setting Rebuild Priority to Low; and setting the Background Surface Scan Interval to 30 seconds (Table 3-1). These tasks can
cause transient dips in read performance and affect the ability of the VOD server to deliver all of the video streams reliably.
Conclusion
Maximizing VOD performance involves many moving parts, such as tuning the Smart Array controller driver, the VOD
application server, the OS, and the file system itself. We continue to examine VOD performance for HP ProLiant systems for
future improvements.