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Question: Explain the components of Intelligent Storage System.

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Explain in brief "Components of Intelligent Storage System"

Marks: 10 M

Year: Dec 2012, May 2013 , Dec 15, May 15

modified 9 weeks ago by gravatar for dpoornima dpoornima0 written 16 months ago by gravatar for Ramnath Ramnath2.0k

An intelligent storage system consists of four key components: front end, cache, back end, and physical disks. Figure illustrates these components and their interconnections. An I/O request received from the host at the front-end port is processed through cache and the back end, to enable storage and retrieval of data from the physical disk. A read request can be serviced directly from cache if the requested data is found in cache.

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Components of an intelligent storage system

1. Front End:

The front end provides the interface between the storage system and the host. It consists of two components: front-end ports and front-end controllers. The front-end ports enable hosts to connect to the intelligent storage system. Each front-end port has processing logic that executes the appropriate transport protocol, such as SCSI, Fibre Channel, or iSCSI, for storage connections. Redundant ports are provided on the front end for high availability. Front-end controllers route data to and from cache via the internal data bus. When cache receives write data, the controller sends an acknowledgment message back to the host. Controllers optimize I/O processing by using command queuing algorithms.

  • Front-End Command Queuing

Command queuing is a technique implemented on front-end controllers. It determines the execution order of received commands and can reduce unnecessary drive head movements and improve disk performance. When a command is received for execution, the command queuing algorithms assigns a tag that defines a sequence in which commands should be executed. With command queuing, multiple commands can be executed concurrently based on the organization of data on the disk, regardless of the order in which the commands were received. The most commonly used command queuing algorithms are as follows:

■■ First In First Out (FIFO): This is the default algorithm where commands are executed in the order in which they are received.There is no reordering of requests for optimization; therefore, it is inefficientin terms of performance.

■■ Seek Time Optimization: Commands are executed based on optimizing read/write head movements, which may result in reordering of commands. Without seek time optimization, the commands are executed in the order they are received.

■■Access Time Optimization: Commands are executed based on the combination of seek time optimization and an analysis of rotational latency for optimal performance. Command queuing can also be implemented on disk controllers and this may further supplement the command queuing implemented on the front-end controllers. Some models of SCSI and Fibre Channel drives have command queuing implemented on their controllers.

2 Cache:

Cache is an important component that enhances the I/O performance in an intelligent storage system. Cache is semiconductor memory where data is placed temporarily to reduce the time required to service I/O requests from the host. Cache improves storage system performance by isolating hosts from the mechanical delays associated with physical disks, which are the slowest components of an intelligent storage system.

Cache is organized into pages or slots, which is the smallest unit of cache allocation. The size of a cache page is configured according to the application I/O size. Cache consists of the data store and tag RAM. The data store holds the data while tag RAM tracks the location of the data in the data store and in disk. Entries in tag RAM indicate where data is found in cache and where the data belongs on the disk. Tag RAM includes a dirty bit flag, which indicates whether the data in cache has been committed to the disk or not. It also contains time-based information, such as the time of last access, which is used to identify cached information that has not been accessed for a long period and may be freed up.

3 Back End:

The back end provides an interface between cache and the physical disks. It consists of two components: back-end ports and back-end controllers. The back end controls data transfers between cache and the physical disks. From cache, data is sent to the back end and then routed to the destination disk. Physical disks are connected to ports on the back end. The back end controller communicates with the disks when performing reads and writes and also provides additional, but limited, temporary data storage. The algorithms implemented on back-end controllers provide error detection and correction, along with RAID functionality. For high data protection and availability, storage systems are configured with dual controllers with multiple ports. Such configurations provide an alternate path to physical disks in the event of a controller or port failure. This reliability is further enhanced if the disks are also dual-ported. In that case, each disk port can connect to a separate controller. Multiple controllers also facilitate load balancing.

modified 9 weeks ago  • written 9 weeks ago by gravatar for dpoornima dpoornima0
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