94-199R1 -- REVISED Minutes of the XOR Study Group Meeting

Paul Hodges phodges at VNET.IBM.COM
Tue Nov 1 17:04:18 PST 1994

on September 12, 1994
Reply-To: phodges at vnet.ibm.com
News-Software: UReply 3.1
X-X-From: phodges at vnet.ibm.com(Paul Hodges)

In the reference, Gerry Houlder wrote
> (5) Multi-controller data validation problem - Paul Hodges (IBM) posed
> the problem of one initiator doing an update write (XDWRITE with an
> XPWRITE to another drive) while another initiator is doing a
> regenerate on the same LBAs.  The regenerate operation could read new
> data from the data drive (because XDWRITE is done or a cache hit on
> new data occurs) and get old data from the parity drive (because
> XPWRITE hasn't happened yet).
> Our conclusion is that this problem is not unique to XOR command
> architectures and can only be solved by having RAID controllers
> cooperate with each other on such activities.  We didn't identify any
> particular implementation rules that should be added to the XOR
> commands.

Unfortunately, I obscured the real problem by presenting it in the
context of multiple controllers.

It is indeed true that the problem of multiple RAID controllers is one
that the controller design must solve.  In fact, most, if not all,
subsystems with multiple RAID controllers apply restrictions so that
the drives of a single parity group are controlled by a single RAID

On the other hand, the Drive-XOR proposal is inherently one of
multiple initiators that do not coordinate their operations.  The
sequence of operations at a drive is not controlled by a single
intelligence, and the condition described in the above example can be
the result of successive operations from the same controller.
Therefore, the data integrity exposure exists in the single controller

There may be scenarios other than the one identified above that can
cause incorrect data to be sent to the application.

Regards, Paul Hodges
:    Telephone:  408-256-6224                     Fax:  408-256-9130    :
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