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

DRAFT -- Revision 2.0
August 22, 1996Page 14
can live with the scheduled aspect of the replacement, especially since the replacement
will occur in much less time than if a T500 had to be shut down and rebooted.
Global hot spare functionality can be simulated with JBODs if a spare spindle is
available on each side of the mirror. With modification of one step, the procedure that
allows quiescent or on-line replacement of a JBOD will instead allow a hot spare to be
reassigned to take over for a failed mechanism. However, this reassignment is not
automatic, and involves the procedure described in Technical HPPA Newsletter # 218.
Advantages and disadvantages of standalone disks with LVM mirroring are:
+ 2- or 3-way mirroring is possible
+ offline backup can be done by splitting off one side of the mirror and activating
it read-only on another system
+ potential for highest read performance due to LVM queuing to the least busy
side of the mirror
+ I/O can be spread across many SCSI interfaces to increase the bandwidth
and do more concurrent I/Os
+ multiple disk controllers reduce chance of controller bottleneck
+ total control of data placement for application performance tuning
+ fewer Single Points of Failure (SPOFs)
each disk has its own controller
there is no master controller that might be a bottleneck or an SPOF
each tray or tower has its own power supply and power cord and can
therefore be powered from different power sources
mirrored trays or towers can be placed on separate SCSI interfaces.
- each individual disk requires a SCSI target address which, in turn, limits
the total disk capacity on the system
- 1 - 3 % additional software overhead due to LVM code paths (only a
consideration if the system is 100 % CPU bound)
- additional cost due to 100% duplication of disk space
- additional rack and floorspace requirements
- use of a hot standby is difficult and not directly supported by LVM
- failed disk replacement might require quiescence of the application
- failed disk replacement might require special operational procedures
Disk Arrays using RAID technology
Disk Arrays are collections of disk drives usually in a common enclosure, each with their
own controller. Typically, one of two methods is used to implement RAID levels with
disk arrays.