APPENDIX
102
RAID controller allows user to choose the task priority (Ultra
Low (5%), Low (20%), Medium (50%), High (80%)) to balance
volume set access and background tasks appropriately. For high
array performance, specify an Ultra Low value. Like volume ini-
tialization, after a volume rebuilds, it does not require a system
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 RAID
storage firmware 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 in-
ternally remap bad sectors without external help using cyclic
redundancy check (CRC) checksums stored at the end of each
sector.
The RAID controller drives perform automatic defect re-assign-
ment for both read and write errors. Writes are always com-
pleted - 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 recov-
erable read error, the correct data will be transferred to the host
