RAID-1 Disk Mirroring / Duplexing



RAID-1 Disk Mirroring / Duplexing

This approach keeps two complete copies of all data, so whenever the computer makes an update to a disk, it can arrange to duplicate that update to a second disk, thus mirroring the original. Either disk can fail, and the data is still accessible. Additionally, because there are two disks, a read request can be satisfied from either device, thus leading to improved performance and throughput. Some implementations optimize this by keeping the two disks 180 degrees out of of phase with each other, thus minimizing latency.

However, mirroring is an expensive way of providing protection against data loss, because it doubles the amount of disk storage needed (as only 50% od the installed disk is available for data storage).

 RAID-1 (Disk Mirroring)
  +-------------+
  |             |
  |  +----------+
  |  |Disk      +-------+-----+-------+
  |  |Controller|       |     | Disk 1|
  |  +----------+       |     +-------+
  |             |       +-----+-------+
  |             |             | Disk 2|
  +-------------+             +-------+


Disk mirroring involves duplicating the data from one disk onto a second using a single controller.

Disk duplexing is the same as mirroring in all respects, except that the disks are attached to seperate controllers. The server can now tolerate the loss of one disk controller, as well as or instead of a disk, without loss of the disk subsystem's availibility or the customer's data. Since each disk is attached to a seperate controller, performance and throughput may be further improved.

 RAID-1 (Disk Duplexing)
  +-------------+
  |             |
  |  +----------+
  |  |Disk      +-------------+-------+
  |  |Controller|             | Disk 1|
  |  +----------+             +-------+
  |  +----------+
  |  |Disk      +-------------+-------+
  |  |Controller|             | Disk 2|
  |  +----------+             +-------+
  |             |
  +-------------+


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