Release Notes

40 Analyzing Dell PS Series Storage with SAN Headquarters | TR1050
The “write penalty” is the additional IOPS needed to protect the disks with RAID. Table 3 shows the values for the
available RAID policies for PS Series arrays.
Table 3 Write Penalty based on RAID
RAID Policy
Write Penalty
RAID 10
2
RAID 50
4
RAID 6
6
Note: RAID Policy is defined on the PS Series member. The Pool may have multiple members at different RAID
policies.
For this example, a RAID 6 solution will be considered which has a write penalty of 6. Plugging in .7 to represent the
reads and .3 to represent the writes we get the non-reduced formula below:
Total disk IOPS = (2358 x 0.7) + ((2358 x 0.3) x 6)
= 4480
The number of disks needed to support this workload will need to be determined. Pick 15K disks from the rule of
thumb in Table 1 (210 IOPS per disk). The last step is to divide the total disk IOPS needed by the rule of thumb IOPS per
disk.
Total disks needed = Total disk IOPS/Rule of thumb IOPS per disk
Note: The rule of thumb IOPS per disk value comes from a best practice formula that is based on disk seek latency,
drive speed, drive type and form factor.
Total Disks Needed = 4480/210
= 22
Because we are using the PS6210XV (15K disks) in a RAID 6 (23 effective disks), this solution is appropriate.
However, consider the pent-up demand effects described below.
3.2.1.2 Pent-up demand
The IOPS per disk are within reason for 15K drives (210 IOPS). However, additional IOPS may be pending due to the
relatively high latency. Pent-up demand is difficult to predict and often requires further detailed analysis to find an
accurate model.
Actual results may vary. As seen in the first example, the load increased to near 7000 IOPS (vs. 3400 IOPS) while
maintaining a reasonable response time. Review the results in Figure 24.