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hxman
2 Bronze

IOP Calculation With Mixed Storage Pool

I would like to calculate the back-end IOPs numbers of a mixed drive storage pool using mixed RAID groups.

example:

Storage Pool 1:

SAS = 15 x 600G 15K drives (Raid 5/4+1)

NL SAS = 8 1TB 7200k drives (RAID 6/6+2)

Estimated IOPs:

15K = 180

7200 = 90

Write penalty:

RAID 5 = 4

RAID 6 = 6

Would you calculate these seperately or as a whole storage pool?

My question:

What is the estimated IOP calculation of the total Storage Pool?

What is the estimated IOP calculation of the SAS disks (15)?

What is the estimated IOP calculation of the NL SAS disks (8)?

Thanks

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10 Replies
RRR
5 Tungsten

Re: IOP Calculation With Mixed Storage Pool

I guess you already know the answer as you have all the numbers!

But specially for you:

  1. What is the estimated IOP calculation of the total Storage Pool?
  2. What is the estimated IOP calculation of the SAS disks (15)?
  3. What is the estimated IOP calculation of the NL SAS disks (8)?
  1. 15 x 180 + 8 x 90
  2. 15 x 180
  3. 8 x 90

The fact that you describe the RAID levels has nothing to do with the amount of back end IOps. It does however have an impact on the front end IOps.

So for example when your hosts has a r/w ratio of 3:1 (75% are reads and 25% are writes) and is doing 1000 IOps this means 750 IOps are reads and will cost you 750 back end IOps, but the 250 front end IOps translate to either 1000 IOps on SAS or 1500 IOps on NL-SAS, so in total your host will generate somewhere inbetween (750 + 1000) and (750 + 1500) IOps. This is between 1750 and 2250. An accurate number is hard since you didn't give me the exact numbers of how much data is being used on SAS and how much on NL-SAS.

RRR
5 Tungsten

Re: IOP Calculation With Mixed Storage Pool

And this estimate is probably on the safe side, since RAM cache also adds extra IOps and depending on the I/O size this value can be higher or lower. If I'm correct the rule of thumb values are based on 4 kB blocks. If the I/Os are larger the number of IOps drops and when they're smaller this number will be higher.

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RRR
5 Tungsten

Re: IOP Calculation With Mixed Storage Pool

So the SAS tier will have 3 x 4 x 600GB capacity = roughly 7TB and 15 x 80 = 2700 IOps

The NL-SAS tier will have 6 x 1TB = roughly 6TB capacity and only 720 IOps.

So roughly a little more than half the capacity can handle 2700 IOps and the other (less than) half only 720 IOps.

The SAS tier is roughly 4 times faster than the NL-SAS tier.

hxman
2 Bronze

Re: IOP Calculation With Mixed Storage Pool

I was under the impression we had to factor in the write penalty when doing these rough calculations?

Example:

SAS = 15 x 600G 15K drives (Raid 5/4+1)

15 x 180 iops = 2700 IOps

Do we not factor in the RAID 5 penalty of 4 somehow?

Thanks

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RRR
5 Tungsten

Re: IOP Calculation With Mixed Storage Pool

Yes you do as soon as you know what the R/W ratio is, but that wasn't your question.

You asked what the back end could handle and you didn't provide me with this ratio, so how can I tell how many writes the back end will need to handle?

RRR
5 Tungsten

Re: IOP Calculation With Mixed Storage Pool

So again: your host is doing 1000 IOps with a R/W ratio of 3:1 which will result in

somewhere between 1750 and 2250 back end IOps, depending on the tier the data is on.

But since the NL-SAS tier only can handle 720 IOps, you're in serious trouble if all I/O is being done on the NL-SAS tier.

RRR
5 Tungsten

Re: IOP Calculation With Mixed Storage Pool

And I did mention the maximum amount of back end IOps are between 720 for NL-SAS and 2700 for SAS, didn't I?

So the NL-SAS is only capable of handleing 720 IOps whatever IO pattern you use. If this is 100% reads, you can do 720 HOST BASED IOps, but if you're doing 100% writes, you can only do 720 / 6 = 120 HOST BASED IOps. Anything inbetween has to do with the ratio I just provided a sample calculation of.

On the SAS tier you can do 2700 IOps in total, which are 2700 HOST BASED IOps when you're doing 100% reads, but this is less if you're doing 100% writes: 2700 / 4 = 675 HOST BASED IOps.

Is this more clear?

hxman
2 Bronze

Re: IOP Calculation With Mixed Storage Pool

Thanks for the information.

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RRR
5 Tungsten

Re: IOP Calculation With Mixed Storage Pool

Always happy to help. If somehow this remains unclear, don't hesitate to ask.

Btw: did you notice there's an "Ask the Expert" area where last August we did a discussion on performance calculations? You can find it here: https://community.emc.com/thread/148025

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