Live Optics | Optical Prime | Latency Basics

Summary: Some notes on Live Optics Latency

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Latency is one of the most important factors to address when designing or optimizing solutions. Reducing latency is a primary objective to ensure optimal application and workload performance.

By definition, this is the amount of time it takes to run a task and it is measured in millisecond values.

It should be noted that latency is a measurement that appears in many places in IT environments. However in Optical Prime, latency is measured from the host level and is relative to the latency of a disk I/O.

Latency of a disk I/O reported from a storage array can differ from the host, as arrays implement latency measurements that are specific to their storage management operations.

However, as a general guideline in a well-designed environment the latency as seen by a storage provider and the host would be very similar.

Note: Optical Prime accurately measures the average latency of a host or hosts with sustained performance of greater than 100 IOPS. High latencies for low periods of IOPS should typically be discounted as they are normally attributable to skew factors with the latency calculation.

Reducing latency is a primary technical objective when architecting a solution or increasing application performance. However, high latency is not always attributable to slow disk or a poorly designed disk subsystem. Latency source can be very difficult to diagnose and could be coming from multiple design issues.

As a general guideline you can quickly make sense of some of the latency feedback

  • High Latency/ Low Queue Depth: Potential Network or Server bottleneck
  • Low Latency/ Low Queue Depth: under-utilized or properly designed resources
  • High Latency/ High Queue Depth: Potential Disk Bottleneck
  • Low Latency/ High Queue Depth: No particular problems
  • High Latency/ Low IOPS: Very low I/O (sub sustained 100 IOPS) can report false positives and can be ignored

Smaller I/O transfer sizes generally want lower latency values.
Very large I/O transfer sizes can create a larger latency that is still acceptable.

For most applications, these values are acceptable.

  • Latency < 10ms is very good
  • Latency < 20ms is acceptable
  • Latency > 40ms is concerning especially if sustained

Read latency is more erratic than write latency due to write caching.

 

Additional Information

If you have any questions, reach out to Live Optics Support at liveoptics.support@dell.com.

 

Affected Products

Active Fabric Controller
Article Properties
Article Number: 000299934
Article Type: How To
Last Modified: 10 Apr 2025
Version:  2
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