Dell PERC 4/SI - Manual (Page 21)

RAID Availability Concept
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.
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 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.
A cold swap requires that you power down the system before replacing a defective hard drive in a disk subsystem.
Sector Re-assignment
Sector reassignment is done automatically by either the drive or the RAID firmware whenever a media defect is encountered.
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 to use them to rebuild failed disks. Manual rebuild is necessary if no hot spares with enough
capacity to rebuild the failed drives are available. You must insert a drive with enough storage into the subsystem before rebuilding the failed drive.
Configuration Planning
Factors to consider when planning a configuration are the number of hard disk drives the RAID controller can support, the purpose of the array, and the
availability of spare drives.
Each type of data stored in the disk subsystem has a different frequency of read and write activity. If you know the data access requirements, you can more
successfully determine a strategy for optimizing the disk subsystem capacity, availability, and performance.
Servers that support video on demand typically read the data often, but write data infrequently. Both the read and write operations tend to be long. Data
stored on a general-purpose file server involves relatively short read and write operations with relatively small files.
Number of Hard Disk Drives
Your configuration planning depends in part on the number of hard disk drives that you want to use in a RAID array. The number of drives in an array
determines the RAID levels that can be supported. See
Table 4 -12 in Assigning RAID Levels for detailed information about the minimum and maximum number
of hard drives supported by each RAID level for each RAID controller.
NOTE: If a rebuild to a hotspare fails for any reason, the hotspare drive will be marked as "failed". If the source drive fails, both the source drive and
the hot spare drive will be marked as "failed".