Calculating IOPS ....

calculating-IOPS.txt

IO Performance:
--------------

NCQ - Native Cmd Queueing - SATA II - 32 cmds

TCQ - Tagged Cmd Queuing - SCSI 2 - 216 cmds possible (64 cmds supported by most disk queues)

- Reordering of cmds when requests for sectors spaced far from one another.

- attempts to reduce disk seek time and rotation latency by optimising order in which sectors are found.

IOPs:

Based on rotational speed, average latency and seek times.

Ave IOPs = 1/(ave latency (in ms) + ave seek (in ms))

Rota­tional speed: 10,000 RPM
Average latency: 3 ms (0.003 seconds)
Average seek time: 4.2 (r) /4.7 (w) = 4.45 ms (0.0045 seconds)
Calcu­lated IOPS for this disk: 1/(0.003 + 0.0045) = about 133 IOPS

....but for the most part you can get by with commonly accepted averages. Of course sources vary .......

Rotational Speed (rpm)	IOPS
	5400		50		80
	7200		75		100
	10k		125		150
	15k		175		210

Multidisk Sets:
--------------

RAID Penalty.....

RAID Level	    Read 	Write

RAID 0		1	1
RAID 1(and 10)	1	2
RAID 5		1	4
RAID 6		1	6

(Total Workload IOPS * Percentage of workload that is read operations) + (Total Workload IOPS * Percentage of workload that is read operations * RAID IO Penalty)

Source: http://www.yellow-bricks.com/2009/12/23/iops/

As an example, let's assume the following:

Total IOPS need: 250 IOPS
Read workload: 50%
Write workload: 50%
RAID level: 6 (IO penalty of 6)

(250 * 0.5)	+ 	(250 * 0.5 * 6)
125		+	125 * 6
125		+	750

Result: You would need an array that could support 875 IOPS to support a 250 IOPS RAID 6-based workload that is 50% writes.

- indicates that the number of disks might be more important than the size (i.e., you would need twelve 7,200 RPM, seven 10K RPM, or five 15K RPM disks to support this IOPS need).


d = num­ber of disks
dIOPS = IOPS per disk
%r = % of read workload
%w = % of write work­load
RF = raid factor (write operations column)


	IOPS 	= 	d * dIOPS
			-------------
			%r + (RF * %w)

where am I sup­posed to get %read and %write from?

sar -d -p will report activ­ity for each block device and pretty print the device name.

	%r 	= 	rd/s
			-----------
			rd/s + wr/s


	%w 	= 	wr/s
			-----------
			rd/s + wr/s