APPENDIX
156
high array performance, specify an Ultra Low value. Like volume
initialization, after a volume rebuilds, it does not require a sys-
tem reboot.
High Reliability
•
Hard Drive Failure Prediction
In an effort to help users avoid data loss, disk manufacturers are
now incorporating logic into their drives that acts as an "early
warning system" for pending drive problems. This system is
called SMART. The disk integrated controller works with multiple
sensors to monitor various aspects of the drive's performance,
determines from this information if the drive is behaving
normally or not, and makes available status information to SAS
RAID controller rmware that probes the drive and look at it.
The SMART can often predict a problem before failure occurs.
The controllers will recognize a SMART error code and notify the
administer of an impending hard drive failure.
•
Auto Reassign Sector
Under normal operation, even initially defect-free drive media
can develop defects. This is a common phenomenon. The bit
density and rotational speed of disks is increasing every year,
and so are the potential of problems. Usually a drive can inter-
nally remap bad sectors without external help using cyclic redun-
dancy check (CRC) checksums stored at the end of each sector.
SAS RAID controller drives perform automatic defect re-assign-
ment for both read and write errors. Writes are always completed
- if a location to be written is found to be defective, the drive
will automatically relocate that write command to a new loca-
tion and map out the defective location. If there is a recoverable
read error, the correct data will be transferred to the host and
that location will be tested by the drive to be certain the location
is not defective. If it is found to have a defect, data will be auto-
matically relocated, and the defective location is mapped out to
prevent future write attempts.
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