As an administrator, you should know how disk I/O limits application performance. Hard disk speed is very crucial in the overall speed of the system. Revolution Per Minute (RPM) directly impacts the latency and disk transfer rate. In this article, we will learn how to calculate IOPS with different speed of hard disks.
The faster the disk will spin, the more it will give read performance, and the slower it spins, the higher will be latency. We have seen 10,000rpm speed with the introduction of Pegasus II SCSI drives.
Below is the table where we can see the IOPS, transfer rate and formula for calculating the number of disk required to get desired workload.
|
|
IOPS |
Data Transfer Rate/ Media Rate |
Formula to Calculate the Minimum Number of Disk Drives to Support Workload |
|
Random I/O (10k RPM) |
125 |
0.5 |
n = (%R + f (%W))(tps)/125 |
|
Random I/O (15k RPM) |
150 |
0.5+ |
n = (%R + f (%W))(tps)/150 |
|
Sequential I/O |
2000 |
50 |
n = (MB/sec)/50 |
Where:
%R = the percentage of disk I/O’s that are reads.
%W= the percentage of disk I/O’s that are writes.
f = 1 for ordinary disks, 2 for mirrored disks, 4 for Raid 5 disks.
Using the above formula, here’s the minimum number of disks required to support a random I/O workload, at 1000 IOPS, 80% read, 20% write on 10K RPM disk drives.
Ordinary disks: (0.8 + 1*0.2)(1000 IOPS)/(125 IOPS/disk) = 8
Mirrored disks: (0.8 + 2*0.2)(1000 IOPS)/(125 IOPS/disk) = 10
Raid 5 disks: (0.8 + 4*0.2)(1000 IOPS)/(125 IOPS/disk) = 13