IBM System Storage DS3500 Introduction And Implementation Manual page 79

Draft Document for Review March 28, 2011 12:24 pm
All write IO's are full stripe writes (no partial stripe writes can be requested)
Write IO's are and sequential and contiguous in nature so no seeks are required.
If any interruptions in this pattern are detected the writes will revert back to the standard full
stripe write model which still gives a benefit to the RAID5 for large sequential writes over the
RAID1.
RAID 6: Data striping with dual distributed parity
RAID 6 (see Figure 3-18) is a RAID level with dual rotational parity that is distributed across
the drives in the array. A RAID 6 array has n+2 redundancy, which means that data remains
accessible to the host application after two concurrent disk drives failures in the array. RAID 6
achieves n+2 redundancy because it calculates two sets of parity information for each block
of data (P+Q) that is striped across the disks. The DS3500 storage subsystem performs the
P+Q parity calculations in hardware.
There is no performance penalty for read operations from a RAID 6 array, but there is a
performance penalty for write operations because two sets of parity information (P+Q) must
be calculated for each write operation. The write penalty in RAID Level 6 may be mitigated by
using battery-backed write caching.
The DS3500 storage subsystem requires a minimum of five drives and supports a maximum
of 30 drives in a RAID Level 6 array.
Figure 3-18 RAID 6
The general characteristics of a RAID Level 6 array are:
It uses disk striping with dual distributed parity: two independent parity blocks per stripe
(P+Q) that are calculated in hardware for each stripe of data.
It has very high data (n+2) redundancy: the array and associated logical drives can survive
the simultaneous loss of two physical disks without losing data.
It requires two sets of parity data for each write operation, resulting in a significant
decrease in write performance. Using battery-backed write cache may mitigate the impact
of this write penalty.
Chapter 3. IBM System Storage DS3500 Storage System planning tasks
7914DS3KPlanning_090710.fm
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