Peripheral Device Architectures: Caching Disks and Disk Arrays

05 Mar

Peripheral Device Architectures

In computer’s hardware language the peripheral devices can be defined as the devices that are connected to the computer in order to get most of the advantage out of it, if you in to detail description of peripheral devices it can be explained as the devices that are optional, and which are not required in principle. In earlier ages when computers were too much expensive and personal computer were very hard to afford, at that time the Motherboard, CPU (Central Processing Unit) and Memory (RAM (random access memory) & ROM (read only memory)) were considered to be the main components of the computer and any other device additionally attached to the computer were considered as peripheral device so this means that the keyboard, mice etc at that time were considered as peripheral devices but now a days generally they are not considered peripheral devices.

The common peripheral devices are microphones, cameras, disk drives, scanners etc. There is a common understanding that among people that the internal devices such as Sound Cards etc are not peripheral because they are added in the computer case which is wrong following is the peripheral devices list for a general idea.

Disk Cache

A disk cache is a portion of a system memory used to cache reads and writes to the hard disk. It may be referred to as the most important type of cache on the PC, because of the greatest differential speed between the layers, that is the system RAM and the hard disk. Disk caching works under the same principle as Memory caching, but instead of using high-speed SRAM, a disk cache uses conventional main memory. The most recently accessed data from the disk is stored in a memory buffer. When a program needs to access data from the disk, it first checks the disk cache to see if the data is there. Disk caching can dramatically improve the performance of applications, because accessing a byte on a hard disk.

While the system RAM is slightly slower than the level 1 or level 2 caches, the hard disk is much slower than the system RAM. Disk caches are usually implemented using software such as DOS’s Smart Drive. When data is found in the cache, it is called a cache hit, and the effectiveness of a cache is judged by its hit rate. Many cache systems use a technique known as smart caching, in which the system can recognise certain types of frequently used data. The strategies for determining which information should be kept in the cache constitute some of the more interesting caching problems in computer.

Peripheral Cache

Other devices can be cached using the system RAM. CD-ROMs are the most common device cache other than hard disks, particularly due to their very slow initial access time. In certain cases, CD-ROM drives are cached to the hard disk, since the hard disk is still faster than the CD-ROM drive.

Finally, we have mentioned and explained the different types of caches. The level 1 and level 2 caches memory are mainly devoted to caching while system RAM is used partially for caching

Disk Array

A disk array is a disk storage system which contains multiple disk drives. It is differentiated from a disk enclosure, in that an array has cache memory and advanced functionality, like RAID and virtualization.

Components of a typical disk array include:

* Disk array controllers

* Cache memories

* Disk enclosures

* Power supplies

Typically a disk array provides increased availability, resiliency and maintainability by using additional, redundant components (controllers, power supplies, fans, etc.), often up to the point when all single points of failure (SPOFs) are eliminated from the design. Additionally those components are often hot-swappable.

Typically, disk arrays are divided into categories:

  • Network attached storage (NAS) arrays
  • Storage area network (SAN) arrays:

– Modular SAN arrays

– Monolithic SAN arrays

– Utility Storage Arrays

  • Storage virtualization


Short for Redundant Array of Independent (or Inexpensive) Disks, a category of disk drives that employ two or more drives in combination for fault tolerance and performance. RAID disk drives are used frequently on servers but aren’t generally necessary for personal computers. RAID allows you to store the same data redundantly (in multiple paces) in a balanced ay to improve overall performance.

There are number of different RAID levels:

Level 0 — Striped Disk Array without Fault Tolerance: Provides data striping (spreading out blocks of each file across multiple disk drives) but no redundancy. This improves performance but does not deliver fault tolerance. If one drive fails then all data in the array is lost.

Level 1 — Mirroring and Duplexing: Provides disk mirroring. Level 1 provides twice the read transaction rate of single disks and the same write transaction rate as single disks.

Level 2 — Error-Correcting Coding: Not a typical implementation and rarely used, Level 2 stripes data at the bit level rather than the block level.

Level 3 — Bit-Interleaved Parity: Provides byte-level striping with a dedicated parity disk. Level 3, which cannot service simultaneous multiple requests, also is rarely used.

Level 4 — Dedicated Parity Drive: A commonly used implementation of RAID, Level 4 provides block-level striping (like Level 0) with a parity disk. If a data disk fails, the parity data is used to create a replacement disk. A disadvantage to Level 4 is that the parity disk can create write bottlenecks.

Level 5 — Block Interleaved Distributed Parity: Provides data striping at the byte level and also stripe error correction information. This results in excellent performance and good fault tolerance. Level 5 is one of the most popular implementations of RAID.

Level 6 — Independent Data Disks with Double Parity: Provides block-level striping with parity data distributed across all disks.

Level 0+1 — A Mirror of Stripes: Not one of the original RAID levels, two RAID 0 stripes are created, and a RAID 1 mirror is created over them. Used for both replicating and sharing data among disks.

Level 10 — A Stripe of Mirrors: Not one of the original RAID levels, multiple RAID 1 mirrors are created, and a RAID 0 stripe is created over these.

Level 7: A trademark of Storage Computer Corporation that adds caching to Levels 3 or 4.

RAID S: (also called Parity RAID) EMC Corporation’s proprietary striped parity RAID system used in its Symmetrix storage systems.

Go to Exercise # 1

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Posted by on March 5, 2011 in Topic 5


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