VCAP – Design: Storage Calculation – Part 2

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This post is a continuation of Storage calculation part 1; in this post we are going to talk about disk types and how we calculate IOPS and Disks for your Design

Note: This post will show basic calculations and will not take into account caching and tiering technologies

Disks types

Now that we’ve covered the raid levels, let’s talk about the disk drives. In the enterprise world we normally see the following disk types:

  • 7.2K RPM NL-SAS or SATA
  • 10K RPM SAS or FC
  • 15K RPM SAS or FC
  • SSD

When we are looking to design a storage system, we need to consider the following:

  • IOPS
  • RAID Write Penalty
  • Application workflow

IOPS = Input/Output operation per second. In spinning disks the numbers of IOPS are well known and they are derived from the disk type and latency.

Based on random I/O, 7.2K drives can generate 75-100 IOPS, 10K drives can generate 125-150 IOPS, depending if you use SATA or SAS technology. 15K drives can generate 175-210 IOPS.

As for SSD’s, there are no fixed numbers like in spinning drives, SSDs can generate more than 5K IOPS. The SSD technology is still booming and the number of SSD IOPS is still rising.

RAID Write Penalty: in the previous post we discussed the raid level and the RAID efficacy. When we put the disks in the array we need to account for the write operation penalty for each RAID level

Use the following table to calculate the total write IOPS for each RAID level:

  • RAID 0 = 1
  • RAID 1 = 2
  • RAID 5 = 4
  • RAID 6 = 6
  • RAID 10 =2

Calculate IOPS in your design:

Let’s assume that in your design requirements you need to provide 100 IOPS per desktop workload and you have to support 200 desktops, not accounting for the capacity at this point, let’s see how we can calculate that.

Also, one of the design assumptions is that each desktop workload is 70% read and 30% write

We will use the following formula to calculate the IOPS:

Desktop workload = (Req IOPS*%READ) + ((Req IOPS*%WRITE)*RAID penalty)

So if we plug the number for RAID 5:

Desktop IOPS = (100*.70) + ((100*.30)*4) = 190 IOPS

If we use 15K FC drives which support 190IOPS

We will need calculate the total amount of IOPS which is 40K and divide it by the disk IOPS

40,000 / 190 IOPS = 211 drives (I rounded the number up)



You will need 211 15K FC Drives in RAID5 configuration for that specific workload.

If you want to change Disk type or the RAID level you just need to adjust the formula.


Thanks for reading,






  • Bjoern Roth

    Hi Mori,
    first of all great blog, very informative and detailed posts. But I think your math here is not entirely correct. First you had the assumption “60% read and 40% write” but you calculated with 70% and 30%. Second when your doing the math with 70% and 30% you get not 200 IOPS, you get 190 IOPS.

    Best regards,

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