Advantech RAID-800S, Installation Reference Manual

Page: 28 / 185

Chapter 2 Functions 1 7
2.10.2 Disk Failure Detection
The subsystem can automatically detect SCSI disk failures. The control-
ler unit monitors the disk activity like the elapsed time on all commands
issued to the disks as well SCSI bus parity error and other potential
problems.
A time-out will reset the disk and retry the command. If the command
time-out occurs again, the disk will fail. Any disk with too many errors
will be destroyed by the controller unit.
2.10.3 Cache Management
The subsystem provides data transfer performance enhancement via its
onboard cache memory. It supports up to 128MB cache memory for read
cache and write cache. Write cache policy is user selectable for maximum
performance of specific applications.
2.10.4 Read Cache
The controller unit’s read cache is always enabled. Its operation is
transparent and requires no user intervention.
2.10.5 Write Back Cache
Write back cache is a caching strategy wherein write operations result in
a completion status being sent to the host operating system as soon as
the cache (not the disk drive) receive the data to be written. The target
SCSI drive will receive the data in a more appropriate time in order to
increase the controller’s performance. To use the write back cache
function, cache must be enabled.
2.10.6 Write Through Cache
Write through cache refers to a cache writing strategy whereby data is
written to the SCSI drive before a completion status is returned to the
host operating system. This caching strategy is considered more secure
because power failure will be less likely to cause any data loss. Howev-
er, write through cache results in a slightly lower performance in most
environments. To use the write through cache function, cache must be
disabled.