Understanding RAID

Using RAID in Verba servers is a strongly recommended option. This topic provides a brief description of each RAID levels with comments on applying them in Verba servers. Verba Media Repository and Recording Server components are different from hard disk point of view. Verba Media Repository runs the database server and the web server, which are transfer-rate-sensitive applications and the Recording Server component runs the recorder engine, which is a write intensive program. Both component require fault tolerance to provide high availability in such mission critical applications like recording. To understand the RAID requirements for Verba servers we provide a comparison for RAID levels too.

The term RAID applies to an architecture that safeguards data - if a disk fails, data is reconstructed. Data is "striped" across several disks. An extra disk is used to store parity information, which is used to reconstruct data.

This architecture ensures that users can always access the data they need at any time.

One side-effect of using RAID, of course, is that the MTBF (Mean Time Between Failure) figures for a RAID subsystem are statistically worse than if only a single drive is involved. If you have a RAID system consisting of, say, four drives and one controller, each with an MTBF of five years, one component of the subsystem will fail, on average, every twelve months. However, against this is the fact that the data held on the RAID subsystem will be safe and it only takes a couple of minutes to replace the faulty drive and for the subsystem to start rebuilding the set.

There are six different levels of RAID and each one is designed to provide greater resilience than the previous level.

RAID comparison

RAID	Advantage	Disadvantage
RAID 0	High performance. No cost penalty - all storage is available for use.	Significantly reduced data availability. No fault-tolerance
RAID 1	Excellent data availability. Higher read performance than a single disk.	Expensive - 50% waste of space. Moderately slower write performance.
RAID 2	Excellent data availability. High performance.	Expensive - requires twice the desired disk space.
RAID 3	Good data availability. High performance for transfer rate intensive applications. Cost effective - only one extra disk is required for parity.	Can satisfy only one I/O request at a time. Poor small, random I/O performance.
RAID 4	Good data availability. High performance for read operations. Cost effective - only one extra disk is required for parity.	Poor write performance. Poor small, random I/O performance.
RAID 5	Good data availability. High performance in request rate intensive applications. Cost effective - only one extra disk is required.	Poor write performance. No performance gain in data transfer rate intensive applications.

RAID configuration recommendations for Verba Recording System

We strongly recommend to use RAID 0 + 1 configurations for all Verba Recording System deployments. This RAID configuration allows to span multiple hard disks and provide mirroring capabilties. This RAID configuration has the best write performance, which is critical for the recording process.