HP Smart Array Controller technology, 4th edition

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The present generation of Smart Array controllers includes rapid rebuild technology for accelerating the
rebuild process. Faster rebuild time helps restore logical drives to full fault tolerance before a subsequent
drive failure can occur, reducing the risk of data lass.
Generally, a rebuild operation requires approximately 15 to 30 seconds per gigabyte for RAID 5 or RAID
6. Actual rebuild time depends on several factors, including the amount of I/O activity occurring during the
rebuild operation, the number of disk drives in the logical drive, the rebuild priority setting, and the disk
drive performance. ACU lets you view the rebuild progress and set the priority for the rebuild operation.
Online spare
Smart Array controllers let you designate an unlimited number of drives as online spares. Multiple arrays
can have the same spare drive as a global spare. Smart Array configuration utilities ensure that SAS disk
drives are only spares for SAS arrays and likewise SATA disk drives for SATA arrays. During system
operation, these spare drives remain up and running but perform no I/O operations. They are available in
case one of the active drives in the array fails. Then an online spare drive may replace the failed drive.
If an active drive fails during system operation, the controller automatically begins rebuilding each fault-
tolerant logical drive onto the online spare; no administrator action is required. Once the rebuild operation
is complete, the system is fully fault-tolerant once again. You can replace the failed drive at a convenient
time. Once you install a replacement drive, the controller will restore data automatically from the failed
drive to the new drive. At that point, the original online global spare will return to standby mode.
Dynamic sector repair
Disk drive media can develop defects caused by variances in the drive mechanisms under normal operating
conditions. To protect data from media defects, HP built a dynamic sector repair feature into Smart Array
controllers.
Smart Array controllers perform a background surface analysis during inactive periods, continually
scanning all drives for media defects. Smart Array controllers can also detect media defects when accessing
a bad sector during busy periods. If a Smart Array controller finds a recoverable media defect, the
controller automatically remaps the bad sector to a reserve area on the disk drive. If the controller finds an
unrecoverable media defect and you have configured a fault-tolerant logical drive, the controller
automatically regenerates the data and writes it to the remapped reserved area on the disk drive.
ECC protection
HP Smart Array cache modules use ECC technology to protect cache data. The ECC scheme generates 8
bits of check data for every 32 or 64 bits of regular data transferred. The cache module uses this
information to detect and correct data errors originating inside the DRAM chip or across the memory bus.
Write caches
Using either the Flash-backed write cache (FBWC) or the battery-backed write cache (BBWC), Smart Array
controllers can acknowledge a data transfer as “complete” before the data is physically stored in the disk
drive. To improve disk write performance, data is temporarily stored in the write cache, which uses DRAM
and is substantially quicker when compared to the disk drive.
Back-up power (FBWC or BBWC) is required for RAID controllers to perform operations such as write-back
cache, array expansion, logical drive extension, strip size migration, and RAID migration.
Recovering data from battery-backed cache
If an u
nexpected server shutdown occurs while data is in the BBWC, Smart Array controllers automatically
signal the memory chips to enter a self-refresh state and the controller moves to battery power or system