Question

Will enabling SCSI Reserve / Release always provide exclusive access to a device?

Answer

Not completely.

SCSI Reserve / Release can be set on a device in the Advanced Options (Settings tab on the Properties page for the device). When enabled, operations on the dervice will be proceeded by an addional "Reserve Unit (16h)" command and followed by a "Release Unit (17h)" command. In a SCSI reference manual, the reservation is descibed like this:

"Reserving a device is a way of ensuring exclusive access to that device from a single initiator for the period of the reservation."

Note that the reservation is done for "a single initiator". This basically means no other HBA can reserve the device until it is released by the HBA that has reserved it already. Knowing that, it is worth mentioning that:

1. Other processes could get access to the device through the HBA that reserved it.
2. If something unexpected happens and processes are crashing, it may be needed to power cycle the device to get rid of the reservation status.

The conclusion is that it is still required to make use of DP's lockname mechanism to ensure a device is only used once at any time. In addition to that, it is also important to make sure no other pocesses can get access to the device in parallel. Zoning can be part of the solution here.

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