Re: Linux Cluster using shared scsi

James Bottomley
2001-05-03T12:54:45Z

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