In the previous chapter, we discussed the lowest layer in the I/O subsystem. Later, in Chapter 4, we begin the discussion of the file system layers that the system users usually see. This chapter is a discussion of a piece that the users rarely see because it falls between these two layers. Located between the disk device driver and the file system layers is an optional product known as the Logical Storage Manager (LSM). The LSM provides a mechanism in Tru64 UNIX for creating and managing storage containers known as logical volumes. A logical volume behaves to the upper layers in the I/O subsystem as either a normal UNIX block or character device. The logical volumes are a powerful method for managing storage in Tru64 UNIX. Not only can volumes be of an arbitrary size, but they also can be organized such that redundancy or performance of the volume is enhanced. LSM supports software RAID levels 0, 1, and 0+1 in Tru64 UNIX versions prior to 5.0. Beginning with version 5.0, RAID level 5 was also added for increased levels of redundancy.

All this power does come at a price. Although the LSM imposes little overhead on the system, it is not free. In addition, management of LSM can be complicated, and there is a steep learning curve to become proficient. Many LSM users report that they set it up once and...