Specifications

SCSI to SATA RAID Subsystem | Administrator’s Manual RAID Functions: Introduction | en 19
Bosch Security Systems F.01U.027.802 | V1 | 2006.11
1.6 Logical Volume (Multi-Level RAID)
1.6.1 What Is a Logical Volume?
Fig. 1.15 Logical Volume
A logical volume is a combination of RAID0 (Striping) and other RAID levels. Data written to a
logical volume is first broken into smaller data segments and striped across different logical
drives in a logical volume. Each logical drive then distributes data segments to its member
drives according to its mirroring, parity, or striping scheme. A logical volume can be divided
into a maximum of eight partitions. During normal operation, the host sees a non-partitioned
logical volume or a partition of a partitioned logical volume as one single physical drive.
The benefits of using a logical volume are achieved by:
1. 1. Extending the MTBF (mean time between failure) by using more redundancy drives
(spare drives).
2. 2. Decreasing the time to rebuild and reducing the chance of data loss caused by simulta-
neous drive failures because drives are included in different drive groups using a multi-
level logical structure.
3. 3. Avoiding the chance of data loss caused by channel bus failure with flexible drive
deployment.
As diagramed below, numerous drives can be included in a logical drive, and one of them is
used for redundancy. By grouping these drives into several logical drives, and then into a logi-
cal volume, the chance of two drives failing in a logical unit is greatly reduced. Each logical
drive can have one or more local spares. A failed drive can be immediately replaced by a local
spare, reducing the risk of losing data if another drive fails soon afterwards.
Fig. 1.16 Logical Drive Composed of 24 Drives