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

Technical HPPA Newsletter # 205, August 1, 1994, "LVM Mirrored Disk
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Recovery".
Technical HPPA Newsletter # 217, May 23, 1995, "Recovery Cookbook".
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DRAFT -- Revision 2.0
August 22, 1996Page 43
performance
backup strategy
total capacity requirements
power source redundancy
total distance
The need for on-line failed disk replacement versus scheduling downtime
Can up to one hour of downtime be scheduled to replace a failed disk? If yes,
then on-line failed disk replacement is not a requirement. Of course,
replacement depends on the availability of a spare disk mechanism and the
knowledge of how to do the replacement.
RAID disk arrays support true on-line replacement of failed disk mechanisms if
they are configured in RAID levels 1, 0/1, 3, or 5 only. The application can
continue to run since the master controller or storage processor limits access to
the failed mechanism. Also, the SCSI busses remain connected and properly
terminated due to the design of the disk array.
With LVM mirrored standalone disks or arrays, it is recommended that the
application be halted so that the simpler replacement procedure can be used.
This also ensures that no I/Os are occurring on the bus at the time of the
replacement. Inadvertent disconnection of the SCSI bus might cause OS and/or
data corruption problems if an I/O was attempted while the bus was
disconnected.
A good discussion of the correct procedure for replacing a failed disk mechanism
in an LVM-mirrored environment can be found in PA NEWS # 205. The
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Australian Response Center has created a cookbook that can be accessed as
described in PA NEWS # 217.
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The need for data redundancy
Data redundancy can be provided in several ways. One must decide first
whether one level of redundancy is sufficient. One-level data redundancy can be
provided with: