6.5.1

Table Of Contents
Problem
Your host is unable to access a LUN, or access is very slow. The host's log files might indicate frequent
path state changes. For example:
Frequent path state changes are occurring for path vmhba2:C0:T0:L3. This may indicate a storage
problem. Affected device: naa.600600000000000000edd1. Affected datastores: ds1
Cause
The problem might be caused by path thrashing. Path thrashing might occur when two hosts access the
same LUN through different storage processors (SPs) and, as a result, the LUN is never available.
Path thrashing typically occurs on active-passive arrays. Path thrashing can also occur on a directly
connected array with HBA failover on one or more nodes. Active-active arrays or arrays that provide
transparent failover do not cause path thrashing.
Solution
1 Ensure that all hosts that share the same set of LUNs on the active-passive arrays use the same
storage processor.
2 Correct any cabling or masking inconsistencies between different hosts and SAN targets so that all
HBAs see the same targets.
3 Ensure that the claim rules defined on all hosts that share the LUNs are exactly the same.
4 Configure the path to use the Most Recently Used PSP, which is the default.
Increased Latency for I/O Requests Slows Virtual Machine
Performance
If the ESXi host generates more commands to a LUN than the LUN queue depth permits, the excess
commands are queued in VMkernel. This increases the latency, or the time taken to complete I/O
requests.
Problem
The host takes longer to complete I/O requests and virtual machines display unsatisfactory performance.
Cause
The problem might be caused by an inadequate LUN queue depth. SCSI device drivers have a
configurable parameter called the LUN queue depth that determines how many commands to a given
LUN can be active at one time. If the host generates more commands to a LUN, the excess commands
are queued in the VMkernel.
vSphere Troubleshooting
VMware, Inc. 65