If your server is unable to access a LUN, or access is very slow, you might have a problem with path thrashing, also called LUN thrashing. Path thrashing might occur when two hosts access the LUN through different SPs and, as a result, the LUN is never actually available.

Only specific SAN configurations in conjunction with the following conditions can cause the path thrashing:

You are working with an active-passive array. Path thrashing only occurs on active-passive arrays. For active-active arrays or arrays that provide transparent failover, path thrashing does not occur.

Two hosts access the same LUN using different storage processors (SPs). This can happen in two ways.

For example, the LUN is configured to use the Fixed PSP. On Host A, the preferred path to the LUN is set to use a path through SP A. On Host B, the preferred path to the LUN is configured to use a path through SP B.

Path thrashing can also occur if Host A can access the LUN only with paths through SP A, while Host B can access the LUN only with paths through SP B.

This problem can also occur on a direct connect array (such as AX100) with HBA failover on one or more nodes.

Path thrashing is a problem that you typically do not experience with other operating systems. No other common operating system uses shared LUNs for more than two servers. That setup is typically reserved for clustering.

If only one server is issuing I/Os to the LUN at a time, path thrashing does not become a problem. In contrast, multiple ESX/ESXi systems might issue I/O to the same LUN concurrently.