Our Tape RAID series provides the ability to perform Tape Striping. The operation is performed at the byte level. Sequential user data bytes are distributed across an even number of array channels with corresponding (even) parity stored on a dedicated channel.
The size of a resulting Tape Striping array data block is determined by the number of channels utilized. Tape RAID or Striping can be distributed across either 2 or 4 different data channels with parity located on a different but dedicated channel. Hence the 2 + 1 and 4 + 1 Tape RAID terminology. For example, with a 4 + 1 Tape RAID operation, every four user data bytes are distributed across four data channels in sequence. The parity byte would become the exclusive-OR of those four data bytes and would be stored on the fifth channel. The resulting data block stored on the Tape RAID drives would be one-fourth its original size in this case.
During the write operation, parity bytes are generated concurrently with the writing of the new user data. The read process is the same except in reverse and can be accompanied by a parity check to ensure data integrity. High speed data transfers are achieved with this configuration because of the combined performance of multiple data channels.
Tape RAID read and write operations in "rebuild" mode are performed when one channel or one drive on a channel is disabled and 'defective' channel read/write options are enabled. Read operations can continue under these circumstances because of the data redundancy provided by the stored parity information. During a "rebuild" read, the missing data is "regenerated" from the stored redundancy. After the drive is replaced or the channel repaired, the data and/or parity stored on that drive can be updated or recovered by a "reconstruction" operation. During rebuild-mode channel write operation the parity is updated to reflect new data.
If "rebuild channel write" is disabled, the array controller reports a write error upon detecting a write error with any one of the drives within a striped set.
If "rebuild channel read" is disabled, the array controller reports a read error upon detecting a read error with any one of the drives within a striped set.
Tape RAID or Striping operations without redundancy are achieved by eliminating all parity related operations outlined above. This approach eliminates the need for an additional drive that holds parity.
For a 2+0 or a 4+0 configuration Tape RAID Level-0 (striping), read and write errors are reported upon detecting a read or write error with any one of the drives within a striped set.
For a 2+2 configuration Tape RAID Level-10, this operates the same as a 2+0 configuration described above plus an identical mirrored 2+0 data set is created simultaneously in parallel. The mirrored 2+0 set is an "EXACT" copy of the primary 2+0 set and can be used for offsite storage with disaster recovery applications.
