Avago Technologies MegaRAID SATA 150-4 (523) - Manual (Page 45)

RAID Availability 2-23
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2.7 RAID Availability
Data availability without downtime is essential for many types of data
processing and storage systems. Businesses want to avoid the financial
costs and customer frustration associated with downed servers. RAID
helps you maintain data availability and avoid downtime for the servers
that provide that data. RAID offers several features, such as spare drives
and rebuilds, that you can use to fix any hard drive problems, while
keeping the server(s) running and data available. The following
subsections describe these features.
2.7.1 Spare Drives
You can use spare drives to replace failed or defective drives in an array.
A replacement drive must be at least as large as the drive it replaces.
Spare drives include hot swaps, hot spares, and cold swaps.
A hot swap is the manual substitution of a replacement unit in a disk
subsystem for a defective one, where the substitution can be performed
while the subsystem is running (performing its normal functions). The
backplane and enclosure must support hot swap in order for the
functionality to work.
Hot spare drives are physical drives that power up along with the RAID
drives and operate in a standby state. If a hard drive used in a RAID
logical drive fails, a hot spare automatically takes its place and the data
on the failed drive is rebuilt on the hot spare. Hot spares can be used for
RAID levels 1, 5, 10, and 50.
Note: If a rebuild to a hot spare fails for any reason, the hot spare
drive is marked as failed. If the source drive fails, both the
source drive and the hot spare drive are marked as failed.
A cold swap requires that you power down the system before replacing
a defective hard drive in a disk subsystem.
2.7.2 Rebuilding
If a hard drive fails in an array that is configured as a RAID 1, 5, 10, or
50 logical drive, you can recover the lost data by rebuilding the drive. If
you have configured hot spares, the RAID controller automatically tries