If your ESXi host is unable to access a LUN, or access is very slow, you might have a problem with path thrashing, also called LUN thrashing.

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.