Friday, June 17, 2011

Introduction to DAS Technology

DAS, in its most basic form, is simply a storage device, such as a hard drive, with a dedicated parallel connection to a server. In larger installations, data is stored to DAS RAID disk subsystems. Servers connect to DAS devices via SCSI (Small Computer System Interface) or Fibre Channel ports capable of transmitting data at a rate of up to 320MB/sec. SCSI is a transfer protocol which uses very fast and efficient granular block I/O commands for transferring data to the DAS device. DAS storage has its share of strengths and weaknesses. Speed and security are two advantages of DAS. With up to 320MB/sec transfer rate, DAS is currently the fastest storage solution available today. DAS is also very secure since it utilizes a direct physical connection to the server, and cannot be spoofed (tricked) into giving access to unauthorized sources.

  • High Performance I/O
  • High Security 
  • Distance Limitation 
  • Low Scalability

The major disadvantages of DAS are its distance restriction, high network overhead, and limited scalability. SCSI device connections cannot typically exceed 12 meters, which means data storage is usually confined to a single room, or even within a single system enclosure.  Network File-Level access to DAS devices is accomplished by sharing the data through the host operating system. Sharing storage in this manner eats up system resources, burdens the LAN (Local Area Network) with storage traffic, and decreases network bandwidth. Another DAS handicap is limited scalability. Since the storage device connects directly to the server, when additional storage is required, more servers must be added. As a result, storage management becomes increasingly complex and costly as demands for data storage grow.

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