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