1. Introduction
N-Port ID Virtualization (NPIV) allows a physical Fibre Channel (FC) adapter on IBM Power Systems to be shared across multiple client LPARs. Each client LPAR uses a virtual FC adapter that maps to a physical FC adapter on the VIOS.
When an application issues an I/O request, it travels through several layers:
• Virtual FC client adapter
• PowerVM hypervisor
• Virtual FC host adapter
• Physical FC adapter
• SAN fabric
• Storage enclosure
Because the I/O path spans multiple components, delays at any point can increase end‑to‑end latency. Identifying the exact layer causing the slowdown can be challenging. To simplify this, AIX provides per‑layer latency insights that help pinpoint abnormal delays and guide corrective action.
2. Enhanced fcstat Capabilities
The fcstat command in AIX/VIOS has been enhanced to display the time spent in each major component of the NPIV I/O stack.
For NPIV client LPARs, fcstat now reports:
• Average read/write completion time
• Maximum latency observed
• Average time spent in:
• SAN
• VIOS
• Hypervisor
• Virtual client adapter
For VIOS partitions or LPARs with a physical FC adapter, fcstat reports:
• Average and maximum I/O completion times
• Average time spent in:
• SAN
• Physical FC adapter
Command Syntax
fcstat -l Duration [Interval] Device_Name
where
Duration = Total time to monitor (seconds)
Interval = Display interval (seconds)
Examples
2.1 Normal I/O Behavior
The example below shows stable I/O completions. Statistics were captured for 20 seconds with updates every 5 seconds.
fcstat -l 20 5 fcs0
2.2 Delayed I/O Completions
In this sample, the statistics clearly show increased delays and identify where the extra time is being spent.
fcstat -l 20 5 fcs0
2.3 VIOS Statistics
When executed on a VIOS (in padmin shell) or an LPAR (as root) with a physical FC adapter, fcstat provides a breakdown of SAN and adapter delays.
fcstat -l 10 4 fcs0
2.4 Error Scenario
In this example, I/O operations are stuck at the host, failing to complete within the expected time. IO operations are monitored for 100 seconds and stats displayed at every 3 seconds interval.
fcstat -l 100 3 fcs0
3. Conclusion
The enhanced fcstat tool provides AIX and PowerVM users with deeper visibility into I/O latency across the NPIV stack. By showing where time is spent, it helps isolate issues more quickly—whether in the SAN, VIOS, hypervisor, or virtual adapters—accelerating root‑cause analysis and resolution.
4. Contacting the PowerVM Team
Have questions for the PowerVM team or want to learn more? Follow our discussion group on LinkedIn IBM PowerVM or IBM Community Discussions