User guide

Maintenance Best Practices for Direct-Attached SCSI Solutions
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5.0 Maintenance of Arrays.
RAID arrays are an industry standard for the protection of important data through
redundancy. This redundancy may take the form of parity calculations that are dispersed
throughout the array. It could also be the simple mirroring of data to maintain a complete
copy that does not require parity calculations to reconstruct the missing elements.
Hard drive media defects and other drive quality issues have steadily improved over time,
even as drive sizes have grown substantially. However, hard drives are not expected to be
totally free of flaws. In addition, normal wear on a drive may result in an increase in media
defects, or “grown defects,” over time. The data block containing the defect becomes
unusable and must be “remapped” to another location on the drive.
If a bad block is encountered during a normal write operation, the controller marks that
block as bad and the block is added to the “grown defects list” in the drive’s NVRAM. That
write operation is not complete until the data is properly written in a remapped location.
When a bad block is encountered during a normal read operation, the controller will
reconstruct the missing data from parity operations and remap the data to the new location.
A condition known as a double fault occurs when a RAID controller encounters a bad block
on a drive in a RAID volume and then encounters an additional bad block on another hard
drive in the same data stripe. This double fault scenario can also occur while rebuilding a
degraded logical drive, leaving the controller with insufficient parity information to
reconstruct the data stripe. The end result is a rebuild failure with the loss of any data in
that stripe, assuming the stripe is in the user data area.
Background Patrol Read
The June/July 2005 firmware and driver updates to PERC 3, PERC 4, and PERC 4e
controllers (not including PERC 3/Di or PERC 4/im) introduce a new feature called
Background Patrol Read. This is a background process that does a read verify across the
physical disk in order to find bad blocks. Background Patrol Read is a media level feature
and does not verify data inside the block or stripe. Background Patrol Read is the most
efficient feature to address media errors.
Run Modes
Because this is a background process, Background Patrol Read is secondary to data I/O and
it efficiently throttles to minimize performance impact
1. Auto
For the majority of the time, this is the recommended mode to use. Auto mode allows
continual checking so that media errors can be detected and data remapped throughout the
product's life.
2. Manual
Manual mode is useful for completing a single scan more quickly than using Auto mode. It
accomplishes this by using different throttling parameters. This "run-once" mode is useful
after making hardware changes to get the hard drives up to optimal condition more quickly.
NOTE:(Your management solution should be updated at the same time as the controller
firmware and driver)