Lenovo ThinkServer RD330 MegaRAID SAS Software User Guide - Page 44

Chapter 2: Introduction to RAID | RAID Availability MegaRAID SAS Software User Guide Table 17: RAID Levels and Capacity (Continued) RAID Level Capacity 10 RAID 10 requires twice as many drives as all other RAID levels except RAID 1. RAID 10 works well for medium-sized databases or any environment that requires a higher degree of fault tolerance and moderate to medium capacity. Disk spanning allows multiple drives to function like one big drive. Spanning overcomes lack of disk space and simplifies storage management by combining existing resources or adding relatively inexpensive resources. 50 RAID 50 requires two to four times as many parity drives as RAID 5. This RAID level works best when used with data that requires medium to large capacity. 60 RAID 60 provides redundancy for two drive failures in each RAID set without duplicating the contents of entire drives. However, it requires extra capacity because a RAID 60 virtual drive has to generate two sets of parity data for each write operation. This makes RAID 60 more expensive to implement. 2.7 RAID Availability 2.7.1 RAID Availability Concept 2.7.1.1 Spare Drives 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 failed 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 drive problems, while keeping the servers running and data available. The following subsections describe these features. You can use spare drives to replace failed or defective drives in a drive group. 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 drives that power up along with the RAID drives and operate in a standby state. If a drive used in a RAID virtual 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, 6, 10, 50, and 60. NOTE: If a rebuild to a hot spare fails for any reason, the hot spare 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." A cold swap requires that you power down the system before replacing a defective drive in a disk subsystem. Page 44