Storage Performance Concepts - Entry # 1

Two key metrics are used to define the performance capabilities of storage arrays and for that matter the underlying physical disk drives themselves. The physical disk drives are what we will be discussing here, since this seems to be the area most people are interested in when evaluating a new storage solution. Notice that I said disk drives, and by this I mean those things with spinning disk in them. Solid State Drives are another matter and we will cover them another time.

IOPS – Refer to the number of read or write IOs that a device can perform per second. In general the IOPS rating provided by the manufacturer (If it is provided at all) is based on random IO and given for both read and write operations. The difference between read and write capabilities is based on the fact that the access time is higher for write operations than it is for read operations. So what if the manufacturer does not provide IOPS information, how can you determine the drives IOPS capabilities?

Here is the generally accepted formula along with an example:

1 / Average Latency in ms + Average Seek Time in ms




Notice that the Fibre Channel and SAS drives provides the same IOPS. This is because with the exception of the interface these drives are the same and the interface has no impact on IOPS. You may also notice that I have not indicated the size of the drives. This is intentional. While there are performance differences between drive generations the drive size has no impact on IOPS. IOPS capabilities are based on the speed of the drive, the RPMs and seek time which when combined are referred to as access time.

MBps – Refer to the Megabytes that can be transferred from a device per second. The MBps rating most often provided by the manufacturer is the Interface Transfer Rate, for example a 4Gb FC drive has an Interface Transfer Rate of 400MBps, a 3Gb SAS device – you guessed it, 300MBps. This can be somewhat confusing since the Drive Transfer Rate or Sustained Transfer Rate – the MBps that the drive itself can actually handle is always less than the Interface Transfer Rate.

In general when evaluating disk drive options from a MBps standpoint you should be concerned with the Sustained Transfer Rate not the Interface Transfer Rate. Interface Transfer Rate is only relevant when you are accessing more than one disk device.

The Sustained Transfer Rate is normally provided as a range such as 198 to 119MBps. This is because the amount of data that can be transferred per second is higher on the outer tracks of the drive surface than it is as you move towards the center.
So when it comes to MBps use the Sustained Transfer Rate as a guideline.
Now that we have covered the metrics used to describe the performance capabilities of a drive, how should you use this information?

Whether you should be concerned with IOPS or MBps depends on how you intend to use the drives – the application requirements and IO profile. Here are some general rules of thumb.

• Applications with a random IO profile such as databases and email servers typically need IOPS more so than MBps.

• Applications with a sequential IO profile such as video or audio streaming, File servers and disk backup targets usually need MBps more so than IOPS.

Again, these are rules of thumb not hard and fast rules. IOPS and MBps are related it is just that an application will typically be limited by one or the other depending on its IO profile.

Simple right? Well not so fast. While the capabilities of an individual drive is important, most of the time you will be grouping these drives into some type of RAID configuration and each RAID configuration uses the available IOPS in a different way. So there’s a little more to consider. Look for information on RAID configurations in entry # 2.

Thanks to Tom Granberry for helping with this post.

Storage Performance Concepts - Entry # 1