Podcast: HDDs efficiency metrics and the workloads they excel at
On this podcast, we speak to Rainer Kaese, senior supervisor for enterprise growth for laborious disk drives (HDDs) for Toshiba Electronics Europe in regards to the use circumstances most suited to HDDs.
Kaese outlines the important thing efficiency traits, particularly in knowledge switch charge and enter/output (I/O) operations per second, and why that makes them most suited to sequential workloads.
He additionally discusses intimately how HDDs work below the hood and the way they evaluate with stable state drives (SSD) by way of I/O efficiency and endurance.
What are the important thing I/O and efficiency traits of laborious disk drives?
There are a number of traits, however just one which is extensively comparable and utterly reproducible, which is the sustained knowledge charge. Meaning the info charge of the outer diameter that the laborious disk drives can ship. That’s additionally crucial one as a result of HDDs are very appropriate for sequential operations. So, you rank and choose and benchmark laborious disk drives by that.
There may be one other metric, which is the I/O operations per second [IOPS], by which HDDs are usually pretty sluggish due to their method of mechanical in search of. And it’s not so comparable. Nonetheless, it’s an essential metric if you wish to benchmark the agility of the laborious disk drive.
Related to that is latency, however latency is determined by many system parameters – principally, how we benchmark HDDs is the sustained knowledge charge and the IOPS.
What does all this imply for the kind of workloads most suited to laborious disk drives?
If the HDD is utilized in a typical operation of sequential knowledge reads and writes, we have now the megabytes per second [MBps] that the laborious disk drive can take or give out. This is determined by the rotational pace and the density of knowledge throughout the tracks.
A tough disk drive spinning at 5,400 revolutions per minute would have a sustained knowledge charge of 170MBps or 180MBpsat its periphery. In the event you proceed studying and writing to the internal [tracks], it drops to round 10MBps. That’s an distinctive case – normally, HDDs we use right now rotate at 7,20rpm, and at 7,200rpm, [the sustained data rate] additionally is determined by the density of the info.
Say ten years in the past, the density was relatively low with 4TB, 6 TB, and 8TB drives, and we had round 20MBps. Right this moment, with 24TB laborious disk drives, the sustained knowledge charge is greater than 300MBps. And, clearly, should you proceed writing or studying to the internal diameter, it drops to the 200MBps vary. The common is about 250MBps.
The opposite [metric] is IOPS. HDDs are usually not actually good for IOPS. These two metrics, MBps and IOPS, should you evaluate them to SSDs, they’re fairly sluggish and fairly poor. SSDs are a number of instances sooner by way of IOPS, even 1000’s of instances sooner.
However we don’t evaluate one-to-one with SSDs since you use one or two [of them]. With HDDs, you employ 4, eight, twelve, as much as lots of within the system. So, though the measures for laborious disk drives are fairly sluggish, should you use lots of them you will get important efficiency.
Again to IOPS. In IOPS, operations per second relies upon how briskly the HDD can discover the observe to be learn or written. And throughout the observe, it additionally has to seek out the small block.
As a mechanical part, that takes time. With the standard IOPS of recent laborious disk drives, they will examine 200 to 220 small recordsdata per second. Sometimes, for this IOPS, the system setup issues.
Once we do comparable measurements, we usually learn and write very small 4K blocks. Additionally, we load the drive with an I/O queue depth of 16. It receives the request for some blocks, it has to seek out the observe, then discover the block throughout the observe. This limits the operation to 220 operations per second. With writes, it’s a little bit bit sooner as a result of it doesn’t have to seek out the info. It is aware of the place to go right away.
Additionally for writes, the HDD can type excellent write request in a method that minimises arm swing to all of the completely different tracks.
Bearing all these parameters in thoughts, what workloads are greatest suited to laborious disk drives?
Any sort of sequential workload. In a sequential workload, the place HDDs run at 300MBps in comparison with SSDs which run at 500MBps or 1000MBps or 2000MBps, that’s not too dangerous. Any sort of sequential workload, video streaming, backup knowledge, is right for a tough disk drive.
If knowledge is written or learn extra randomly, it turns into very sluggish on a single laborious disk drive. Larger programs with many laborious disk drives normally pack the info into bigger chunks based mostly on SSD or reminiscence operations after which write them sequentially to the bigger HDDs.
Additionally, HDDs don’t have any endurance limitations. This makes them ultimate – and doubtless additionally the one appropriate storage media – for video or surveillance knowledge as a result of there you have got many overwrites that you just can’t do with SSDs.
The extra sequential your workload is in studying and writing, the higher suited it’s to HDDs and the extra efficiency you will get out of that media.
