Areca 24/4 Internal/External Port Pcie 3.0 12 Gb/s SAS/SATA Raid Controller 2Gb Cache - Manual (Page 253)

253
APPENDIX
The RAID controller drives perform automatic defect reassign-
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 re-
coverable 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 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 will be flagged as being
potentially defective. A subsequent write to that location will
initiate a sector test and relocation should that location prove
to have a defect. Auto Reassign Sector does not affect disk
subsystem performance because it runs as a background task.
Auto Reassign Sector discontinues when the operating system
makes a request.
• Consistency Check
A consistency check is a process that verifies the integrity of
redundant data. To verify RAID 3, 5, 6, 30, 50 or 60 redun-
dancy, a consistency check reads all associated data blocks,
computes parity, reads parity, and verifies that the computed
parity matches the read parity.
Consistency checks are very important because they detect
and correct parity errors or bad disk blocks in the drive. A
consistency check forces every block on a volume to be read,
and any bad blocks are marked; those blocks are not used
again. This is critical and important because a bad disk block
can prevent a disk rebuild from completing. We strongly rec-
ommend that you run consistency checks on a regular basis —
at least once per week. Note that consistency checks degrade
performance, so you should run them when the system load
can tolerate it.