Stripes and Mirrors

Disk Mirroring

Disk Mirroring, also called Volume Shadowing, is a technique for improving the fault tolerance of magnetic disk storage. Data is written to two (or more) duplicate disks simultaneously. Thus, if one of the disk drives fails the system can immediately switch to the other disk without any loss of data or any interruption of service.

Every write or update is done to both of the physical disks. Any delete operation is also done to both disks at the same time, so mirroring provides no protection from "The Oops Factor."

Here Physical Disk 1 and Physical Disk 2 comprise a mirror set (or shadow set). Because every write or delete operation is done to both physical disks at the same time, the contents of Physical Disk 1 are always identical to the contents of Physical Disk 2. Users and applications reference/access the logical mirror set rather than either of the actual physical volumes. Storage capacity is equal to the size of one of the physical disks.

Disk Striping

Disk Striping is a technique for spreading data across multiple physical disk drives to improve performance. Striping can speed up operations that read data from disk storage by allowing parallel data transfers from different physical disks (i.e., simultaneous read operations from separate disks).

Striping offers no protection from hardware failure. If there is a failure of any one physical disk that is a member of a stripe set, then the entire stripe set (virtual disk) fails.

Here Physical Disk 1 and Physical Disk 2 make up a stripe set, a virtual disk with a storage capacity equal to the sum of the capacities of its members. Users and applications reference/access the single virtual disk rather than any of its individual physical disk members. If any one member (i.e., physical disk) fails then the entire virtual disk fails.

However, what if you had a stripe set where the members were not physical disks but rather mirror set virtual disks?

Fault tolerance can be provided within a stripe set if its individual members are mirror sets rather than single physical disks. Here Physical Disks 1 and 2 make up Mirror Set M1, and Physical Disks 3 and 4 comprise Mirror Set M2. The virtual disks M1 and M2 together form a stripe set that creates the virtual disk S1.

The stripe set virtual disk S1 survives even if any single physical disk member within it fails. Users and applications reference only the virtual disk S1 and are not concerned about the individual physical disks within the stripe set.

Assuming, for example, that all four physical drives (Disks 1, 2, 3, and 4) are 1 GB in capacity, the total storage capacity of the virtual device S1 is 2 GB. Disks 1 and 2 are a mirror set with a total capacity equal to one of the disks (i.e., 1 GB). And, disks 3 and 4 are also a mirror set with a total capacity of 1 GB. Thus the two mirror sets which make up the stripe set give the stripe set a capacity of 2 GB.

Important Note: For the sake of simplicity and clarity, the stripe sets and mirror sets in these examples consist of only two physical disks each. Actual mirror sets may consist of more than two physical disks thereby increasing fault tolerance. And, actual stripe sets may consist of more than two disks to provide increased storage capacity.