3Ware 9550SXU-12 User Manual page 175

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Note:
Not verifying the unit periodically can lead to an unstable unit and may cause
data loss.
It is strongly recommended that you schedule a verify at least 1 time per week. You
can take advantage of the the Auto Verify and Basic Verify Schedule to accomplish
this.
What Verification Does
For a RAID 1 or RAID 10 unit, a verify compares the data of one mirror with
the other. For RAID 5, RAID 6, and RAID 50, a verify calculates parity and
compares it to what is written on the disk drive. For non-redundant units, it
performs a media scan.
Verification checks each sector on a drive. This is important, because day-to-
day use of the media may leave many sectors on a drive unused or unchecked
for long periods of time. This can result in errors occurring during user
operation. Periodic verification of the media allows the disk drive firmware to
take corrective actions on problem areas on the disk, minimizing the
occurrence of uncorrectable read and write errors.
Verifies can be performed through either 3BM (BIOS) or through 3DM. In
addition, they can be scheduled to run at preferred times, through 3DM or
through the CLI, or can be run automatically during the Verify schedule
window, if scheduling and the Auto Verify feature are enabled.
Verification of Non-Redundant Units
Verification of non-redundant units (single disks, spares, and RAID 0 units)
read each sector of a drive, sequentially. If a sector can't be read, it is flagged
as unreadable, and the next time the controller writes to that location, the
drive reallocates the data to a different sector.
Verification of Redundant Units
Verification of redundant units reads each sector, working from lowest block
to highest block. If verification cannot read data in a sector, dynamic sector
repair is used to recover the lost data from the redundant drive or drives; this
recovered data is written to the problem sector. This forces the drive to
reallocate the defective sector with a good spare sector.
If the verify unit process determines that the mirrored drives are not identical
or the parity is not correct, the error is corrected. For RAID 1 and 10, this
involves copying the miscompared data from the lower port(s) to the higher
port(s) of the mirror. For RAID 5, RAID 6, and RAID 50, this involves
recalculating and rewriting the parity that was incorrect. AEN 36 (
detected and fixed data/parity mismatch"
Background Tasks
"Verify
) is posted to the Alarms page.
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