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INTRODUCTION
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1.5 High Reliability
1.5.1 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
S.M.A.R.T. 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 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.
1.5.2 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 is
the potential of problems. Usually a drive can internally remap
bad sectors without external help using cyclic redundancy check
(CRC) checksums stored at the end of each sector.
SATA drives perform automatic defect re-assignment 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 location and map out the
defective location. If there is a recoverable read error, the cor-
rect 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 automatically relocated,
and the defective location is mapped out to prevent future write
attempts.
In the event of an unrecoverable read error, the error will be
reported to the host and the location agged as potentially defec-
tive. A subsequent write to that location will initiate a sector test
and relocation should that location have a defect. Auto Reassign
Sector does not affect disk subsystem performance because it