Messages in this thread | | | Subject | Re: Linux Cluster using shared scsi | Date | Thu, 03 May 2001 08:53:42 -0400 | From | James Bottomley <> |
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dledford@redhat.com said: > Correct, if you hold a reservation on a device for which you have > multiple paths, you have to use the correct path.
As far as multi-path scsi reservations go, the SCSI-2 standards (and this includes the completion in the SCSI-3 SPC) is very malleable. The standard is very explicit about multi-port targets but vague about whether initiator means one port of the initiator or all ports.
If you interpret the standard most stricly, you can read that acquiring a reservation on one port locks everyone (including you) out of all the other ports. However, vendors of symmetric active multi-port arrays tend rather to frown on this interpretation. They take the view that a reservation acquired by an initiator on one port ought to allow that initiator access on all the other ports (otherwise what's the point of being symmetric active). This can only be done if you make assumptions about how you identify the same initiator on a different port. EMC, I believe, assumes that the initiator always has the same SCSI ID. Note, however, that the same SCSI ID assumption will fail in a multi-path point-to-point configuration where all initiators could have the same ID.
This rather unmanageable state of affairs is the reason for SCSI-3 reservations. Since each initiator is now known by a key, you can always be sure to grant access correctly in a multi-ported environment.
The bottom line is that if you use SCSI-2 reservations in multi-port environments, the results are extremely vendor specific.
James
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