I had a problem with MSCS cluster when I rebooted one of the ESXi host & found that ESXi host was stopped responding at the time of boot / discovery.
Then I got to know that there were 32 RDM were mapped to VM which were causing the issue
What are perennial reservations and why are they needed? Whenever a LUN is participating in a MSCS cluster, the active node has ownership of that device(s) using permanent SCSI reservations. Now, whenever you rescan for devices on an ESXi host, or are booting an ESXi host, the host tries to query all devices that it can see, including the devices used for MSCS. Now, I'm not exactly sure what takes place during the query process, but because the MSCS device(s) are already have a permanent SCSI reservation, the ESXi query to the device fails, and will continue to fail until the host decides to move on
To solve the problem of of ESXi trying to query MSCS owned devices, a flag has been introduced on a device called Is Perennially Reserved. By default this flag is set to false. By setting this flag to true it lets the ESXi host know to essentially, NOT query this device during rescans/boot time. Here's VMware KB 1016106 that describes that problem/resolution
Read the KB from VMware which talks about more
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