Subject: Storage Network Management and Retrieval

Difficulty: Medium.

Marks: 10M

Architecture-Related Limitations

Storage network is easier and more flexible to manage than server-centric architecture due to the separation of servers and storage devices. Free hard disk capacity is not enough to fulfil the potential of storage-centric architecture. Without storage virtualization, direct connection is available between the storage device and the server.

The replacement of storage devices requires extra budget where more powerful devices can transfer data from old media to new media.

System administrators have to face complex challenges to implement these storage devices for 24 X 7 environments without application failure.

Without storage virtualization, each and every change made to the storage resources also requires changes made to the operating system and application running on the server.

Consider a storage network which has two servers, that is, server A and server B, with disk space allocated on a disk subsystem. If the disks on server A have been filled, this server cannot simply share the free storage space on the drives of server B, because the volumes on a disk subsystem are still statically assigned to server B and no suitable mechanisms have been implemented in the disk subsystems for sharing block-level resources. Hence, it is possible to assign the same volume manager to different servers on most disk subsystems.

The data inconsistencies that could be a result of such disk sharing are not rectified by the disk subsystem itself. Hence, additional new storage is required to be purchased in order to solve space problems of server A, though the capacity allocated to server B has not yet been used up. Storage virtualization software can realize the sharing of resources in a storage network. However, the complexity of the software to be maintained will increase the administrative cost.

Figure 1: Free storage spaces of server A and server B. Server A storage space is full and cannot utilize free server B storage space.

Implementation-Related Limitations

The vendors of disk subsystems provide device drivers for their storage devices which provide all functions, such as handling multiple paths between a server and the storage device. Unfortunately,these disk subsystem device drivers only work at the disk subsystem level and this makes the matter worse.

The installation of the device drivers for different disk subsystems on one computer at the same time is not supported. This incompatibility between disk subsystems device drivers means that each server is linked to a specific disk subsystem model or a specific range of disk subsystem models, depending upon the device driver.

Figure 2: Device driver incompatibilities. Server can only use storage capacity of same disk subsystem.