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How to Calculate IOPS (I/O’s per Second)

February 10th, 2009 No comments
 

IOPS (I/O’s per Second,
or iostat "tps")

Data Transfer Rate
(MB/sec)

Minimum Number of Disk Drives to Support Workload

Random I/O (10k RPM)

125

0.5

n = (%R + f (%W))(tps)/125

Random I/O (15k RPM)

150

0.5+

n = (%R + f (%W))(tps)/150

Sequential I/O

2000

50

n = (MB/sec)/50

Where:
%R = the percentage of disk I/O’s that are reads.
%W= the percentage of disk I/O’s that are writes.
f = 1 for ordinary disks, 2 for mirrored disks, 4 for Raid 5 disks.
Assumes data is distributed evenly across all disk drives.

 

Using the above formula, here’s the minimum number of disks required to support a random I/O workload, at 1000 IOPS,  80% read, 20% write on 10K RPM disk drives.

 

Ordinary disks:  (0.8 + 1*0.2)(1000 IOPS)/(125 IOPS/disk)  = 8
Mirrored disks:  (0.8 + 2*0.2)(1000 IOPS)/(125 IOPS/disk)  = 10
Raid 5 disks:      (0.8 + 4*0.2)(1000 IOPS)/(125 IOPS/disk) =  13

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