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. |
