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   What is
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   RAID, RAIT or
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What is Striping?
Striping Solutions
dot.gif (333 bytes) Tape Drives
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Description
dot.gif (333 bytes) Operating Modes
dot.gif (333 bytes) Theory of Operation
dot.gif (333 bytes) Reconstruction
dot.gif (333 bytes) Custom Variations

 

Tape Striping Example using 5 independent Tape Drives
Striping 4+1 Animation

Operating Modes

Striping - across up to 4 drives with or without an additional parity drive. Data transfer rate is increased by a factor of up to four respectively.
Operating Mode Number of Tape Drives Tape
RAID
Level
Description
Fault
Tolerant
Striping 2 2 0 2 Drive Striping No
Striping 2+1 3 3 2 Drive Striping with Parity Protection Yes
Striping 2+2* 4 10 2 Drive Striping + Duplicate Copy Yes
Striping 4 4 0 4 Drive Striping No
Striping 4+1 5 3 4 Drive Striping with Parity Protection Yes
*Based on Ultera's proprietary Tape RAID ASIC design. 

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Theory of Operation

Ultera's virtual Tape RAID Level-3 operation is referred to as "byte striping". 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 the resulting array data blocks are determined by the number of channels utilized. Tape RAID Level-3 data 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 Level-3 terminology. For example, with a 4 + 1 Tape RAID Level-3 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 Level-3 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 Level-3 "crippled" read and write operations are performed when one channel or one drive on a channel is disabled and crippled 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 crippled 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 crippled channel write operation the parity is updated to reflect new data.

If "crippled 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 "crippled 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 Level-3 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, 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.

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Custom Variations

As described above, Tape RAID Level-0 or Tape RAID Level-3 operations are referred to as "byte striping".  The following table lists custom Striping mode variations available for Ultera's OEM customers:
Operating Mode Number of Tape Drives Tape
RAID
Level
Description
Fault
Tolerant
Striping 3+0* 3 0 3 Drive Striping No
Striping 3+1* 4 3 3 Drive Striping with Parity Protection Yes
Striping 2+2* 4 10 2 Drive Striping with Mirror Copy Yes
Striping 5+0* 5 3 5 Drive Striping No
*Based on Ultera's proprietary Tape RAID ASIC design

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