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As larger disk drive capacities are achieved (such as with 300 GB), the total amounts of data which would need
to be recovered increases. The recovery time increases commensurately with the amount of data being
recovered. As the recovery time lengthens, some customers are concerned that if a second disk drive fails, their
ability to recover their data would be lost.
RAID 6 is similar to RAID 5 as blocks of data and parity information are striped across an array of drives,
except that in a RAID 6 implementation there are two sets of parity information striped across an array of
drives. This duplication is solely to improve fault tolerance – RAID 6 can handle the failure of any two drives
in the array while other single parity RAID levels can handle at most one fault. Performance-wise, RAID 6 is
generally slightly worse than RAID 5 in terms of write operations due to the added overhead of more parity
calculations, but may be slightly faster in random read operations due to the spreading of data over one more
disk. The StorageTek 9900 will support RAID 6 sets as 8-drive stripe depth (6 data 2 parity drives).
For these reasons additional information and focus is provided below.
The 300GB 15K RPM HDD (288.20GB unformatted, raw) achieved general availability May 3, 2005 for the
ST9990. This drive is also used in the ST9990 V and ST9985V. This disk drive extends the maximum internal
storage capacity of the ST9985V to 69.2 TB (un-formatted, raw).
Additional RAID levels and striping sizes are under consideration and further information will be provided
when available.
The ST9985V supports RAID-1, RAID-5, and RAID-6 array groups.
RAID-1. illustrates a sample RAID-1 (2D+2D) layout. A RAID-1 (2D+2D) array group consists of two pair of
disk drives in a mirrored configuration, regardless of disk drive capacity. A RAID-1 (4D+4D) group* combines
two RAID-1 (2D+2D) groups. Data is striped to two drives and mirrored to the other two drives. The stripe
consists of two data chunks. The primary and secondary stripes are toggled back and forth across the physical
disk drives for high performance. Each data chunk consists of either eight logical tracks (mainframe) or 768
logical blocks (open systems). A failure in a drive causes the corresponding mirrored drive to take over for the
failed drive. Although the RAID-5 implementation is appropriate for many applications, the RAID-1 option on
the ST9985V is ideal for workloads with low cache-hit ratios.
*Note for RAID-1(4D+4D): It is recommended that both RAID-1 (2D+2D) groups within a RAID-1 (4D+4D)
group be configured under the same DKA pair.
Just the Facts
October 2007
46
Sun Confidential – For Internal Use and Authorized Partner Use Only