Choosing the Right Disk Technology in a High Availability Environment DRAFT Version 2.0, August 1996

DRAFT -- Revision 2.0
August 22, 1996Page 24
up to 12 disk modules of 2.1 GB each
active hot spare disk module
on-line capacity expansion
on-line replacement of failed disk modules
redundant dual active controllers with automatic failover
ECC cache
redundant fans
optional redundant power supply and power cord
on-line replacement of controllers, fans and power supplies
Cost, capacity and the features listed above make the AutoRAID the practical choice in
many situations where 50 - 200 GB of storage is required and where performance and
application access pattern knowledge is not available. The RAID level in AutoRAID
cannot be configured. The AutoRAID unit determines the best RAID level for the data
depending upon access patterns and the configured limit of RAID 1 storage.
When ordering AutoRAID units that will be mounted in cabinets, it is important that the
cabinets be ordered with a second power distribution unit (PDU). This second PDU
provides a second power strip inside the cabinet and a second power cord that can be
plugged into a separate power source. This configuration removes power as an SPOF.
Disk storage is assigned to a SCSI LUN according to desired capacity. Adding new
disk mechanisms does not require a reload of the data since new disks may be
assigned to an existing LUN online, a feature unique to AutoRAID.
The AutoRAID can be configured with up to 12 disk mechanisms assigned to a
combination of RAID 0/1, RAID 5 and hot spare. Internally, there are four SCSI busses
with up to 3 storage modules on each bus. There is no control possible over data
placement on physical disks. The LUNs are defined according to the amount of disk
space desired.
The amount of RAID 0/1 space is dynamic, limited by the amount of unallocated space
for LUNs, and is allocated according to the data access patterns of the application with
a minimum of 2 GB. The most active data is kept in RAID 0/1 for best performance.
Data that is less frequently accessed is kept in RAID 5 for lower cost. The administrator
controls only the total amount of space available for RAID 1.
The AutoRAID comes with two controllers that are active and redundant. That is, both
controllers can be used to access the data. In the event that one controller fails, the
other controller can be used to access all of the data in conjunction with the PVLinks
feature of the operating system, described in a previous section. AutoRAID does not
have the performance impact that the HADA has when switching between controllers
for the same LUN. Note that PVLinks does not currently allow concurrent access to the