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IOPS calculation

Written by on . Posted in How to

How to calculate the average IOPS writen to disk? Good question but not always easy to answer.

Accoridng SNIA's Dictionary the write penalty is inherent in RAID data protection techniques, which require multiple disk I/O requests for each application write request, and ranges from minimal (mirrored arrays) to substantial (RAID Levels 5 and 6). Many RAID array designs include features such as write-back cache specifically to minimize the write penalty.

Write penalty

RAID 5 (4+1) = 4

RAID 1 (1+1) = 2

RAID 10 (1+1) = 2

RAID 10 (2+2) = 2



In a RAID1 environment the formula is:

h = disk IOPS / (r+2w)

h= 520 / 0.7+(2x0.3) = 520 / 0.7+0.6 = 520 / 1.3 = 400


In my case, with 4 x 180 IOPS, you have 720 disk IOPS available. To get host IOPS you need to know your read/write rate. If it is 60/40 read/write, it is

h = 720 /(0.6 + 2*0.4)

h = 512 IOPS


Note the above equation is derived from the equation for disk IOPS from host load (h = host IOPS, r = read%, w = write%) which for RAID 1 and RAID 1/0 is as follows:

r*h + 2*(w*h) = diskIOPS

example r = 60% and w = 40%, for 10000 host IOPS

.6*10000 + 2 * 0.4 * 10000= diskIOPS

6000 + 2 * 4000= diskIOPS

14000 = diskIOPS





In this example I use the following specs:

- EMC CLARiiON CX4-240
- 450 GB 15K7 FC disks (ST3450857FC)
- (avg. 180) I/Os per second per disks
- approximate read/write mix is 60/40
- HostIOPS = 520
- Legend: h= host IOPS / r= read% / w=write%


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