Thursday, March 29, 2012

VMware ESXi 5.0 installer doesn't always show LUN 0

If you have ESXi hosts which have hit or are close to hitting the VMware path limit maximum (2048 paths per host), it is possible that the LUN (Logical Unit Number) you want to use for boot (LUN 0) will not be listed by the ESXi installer. This affected me because I incorrectly assumed that LUN 0 had to be enumerated.

In reality, LUNs will enumerate in any order the storage system feels like responding in. From my experience, this is non-deterministic. LUN 0 may be enumerated first, or last, or in the middle. And just because a subset of LUNs is enumerated once, does not mean that the same subset will be enumerated next time!

This issue is more likely to impact you if you have many paths per LUN. The solution is to either:

  1. Reduce the amount of LUNs you have through LUN consolidation. Have fewer, larger LUNs. Take advantage of increased LUN size maximums.
  2. Change to an internal boot device. SD card boot, or internal disks.
  3. Start splitting clusters. Redistribute which LUNs get presented to which hosts.

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