Text of High Availability doc "Propsoed Controller Failover Profile"
Steve Sicola...Array Controller Engineering 522-2268
sicola at peaks.ENET.dec.com
Thu Sep 7 15:16:15 PDT 1995
X3T10 Document xxxxxxx
SCSI-3 Fault Tolerant Controller Configurations
utilizing SCC & New Event Codes
The purpose of this document is to profile the use of SCC with new ASC/ASCQUs
that would allow Operating System Drivers to be written to generically support
fault tolerant controller configurations in open systems environments. This
document further describes the implicit assumptions about the relationship
between controllers in a fault tolerant controller configurations so that
deviations from the SCC model are not needed.
Within the confines of the SCSI-3 SCC (Standard Controller Commands) lie the
tools in order to TregisterU and TreportU the existence of fault tolerant
controller configurations that contain two or more controllers sharing the
same storage devices and paths to those storage devices. The SACL within each
controller in a fault tolerant configuration is logically presented as a
single SACL with multiple ports. Specifically, the two controllers must
present the exact same configuration when SCC TReportU commands are presented
to either controller.
The single controller with multi-ports works based upon two assumptions that
are typically no problem for controllers in fault tolerant configurations to
comply with. These assumptions are
1. Controllers in a fault tolerant configuration communicate with each other
to relate changes in state or configuration
2. Controllers in a fault tolerant configuration will always have report the
same configuration to all attached hosts.
3. Controllers supported will include those with single or multiple host
interfaces, single or multiple shared device interfaces, and those that
contain separate or integrated communication between controllers in a fault
In order for controllers to be in a fault tolerant configuration, this
assumption is a very logical conclusion so that failover can be graceful, so
as not to cause data corruption upon failover, reduce the time of failover
(time to LUNs appearing on the alternate controller).
Utilizing SCC, the Attach Component Device command will be used by host to
create an attachment between controllers to create or add to a fault tolerant
controller configuration. The configuration may also be setup by other means
available to the controllers (external user interfaces) in which case the use
of Create Attachment command is unnecessary.
In order for host computers to recognize fault tolerant controller
configurations, the Report Component Device Attachment command must be used to
interrogate controllers to determine that multiple paths do exist to the same
storage devices. The controllers must also share the use of LUN0 on every
controller in the same controller configuration. LUN0 on each controller will
report the same configuration. This keeps the subsystem consistent.
Furthermore, any host or external user interface configurations entered on one
controller MUST also be relayed immediately to all other controllers in the
same fault tolerant controller configuration.
The Attach Component command needs one more aspect (noted below in open
issues) in order to adequately cover all possible actions of a controllers in
a fault tolerant configuration. That aspect is controller configuration
naming. The naming must cover the needs of the overall system the
configuration is attached to. The naming must handle a change in membership,
either from an addition to the configuration, a deletion from the
configuration, or from a replacement in configuration (after failure). The
name must be unique with a system installation, but not necessarily world-wide
unique. The name must essentially be a controller configuration ThandleU that
can be used by any host operation system to key off of in order to handle
multiple paths to the same devices or LUNs. The events of failover and
failback are defined as follows:
1. Failover - The failure of a controller to continue accessing of
attached devices for whatever reason resulting in the failure to serve host
requests. Failover to a surviving controller means that the surviving
controller must be able to restore access to attached devices and restore
request service to hosts. The action in some cases is required to be as quick
2. Failback - The restoration to active service by a previously
failure of a controller. The controller that restored access originally for
the failed controller may signal attached hosts that the resource has returned
and is now available for service and will continue to service the attached
devices unless directed otherwise (reserve and release activities).
The failover and failback events noted above can be achieved in many ways
within the controller and are outside the scope of this profile, except in the
area of event codes denoted the occurrence of the failover or failback event.
In these cases, two new ASC/ASCQUs are required to support this possibility.
These two event codes would denote Failover and Failback. They would be used
by the controller taking over for a failed controller or that the failed
controller has returned. The uses for these event codes are to allow for much
more pro-active host involvement by port drivers instead of the use of
timeouts. Timeouts based event actions do work adequately, but in some cases
do NOT meet the needs of highly available customer installations. The needs in
these cases are speedy failover and speedy failback in order to regain access
in the event of failure and to regain performance in the event of controller
Additional Requirements & Issues for SCSI-3
The event codes would be used with Unit Attentions or AENs from the chosen
TFailover/FailbackU Controller. The choice of which controller in a
multi-controller fault tolerant configuration is outside the scope of this
profile because the mechanisms to allow choice are here with the use of the
new event codes. The unique identification of a Controller Configuration is
required. It does not need to be a World-wide type name, rather a
Tsystem-wideU unique name that can cover the configuration across the hosts to
which it is attached. This name will survive any controller failures and
replacements, so as to not rely upon the serial number of the controller or
any packaging specific addresses. Hosts will TknowU about the Controller
configuration by name, simplifying any mapping between multiple access paths
to the same data.
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