We are currently using an HDD for our NetApp storage for our VMs. We're considering replacing them with SSDs, but many disk manufacturers have different performance. What range of latencies can we expect from SSDs versus HDDs? How much better are SSDs on average?
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2 Answers
The latencies of an SSD vary hugely, but in terms of IOPs (which us probably the term you want to google)you are looking at an increase in the order of 2-5 magnitudes. (Ie 100 iops for HDD, starting at more then 10000 + for an SSD).
Have a look at https://en.m.wikipedia.org/wiki/IOPS
SSDs are like a night and day difference. I would only use hdds for backups and unimportant archival data - like videos, music collections. A heavily swapping HDD can render a system unuseable - while it will just be a bit sluggish on SSD.
Speedwise SSDs are at least 5 times as fast in sequential reads and writes - which represents the closest benchmark
You did not ask, but SSD is also over 5 times as reliable as hdd (but unlike hdd more likely to fail completely and without warning - so RAID is still important)
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If you reread my answer, you will see that I did. SSDs start at 5 times more throughput and go up from there– davidgoCommented Mar 17, 2019 at 6:20
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another question please , lets say I have HHD disks , and by dmidecode , I get all disks details , can we according to the info from dmidecode find the iops? , if not maybe other linux command ? Commented Mar 17, 2019 at 6:40
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BTW - you said " latencies of an SSD vary hugely" but how it can be while iops is very good and latency is hudely? Commented Mar 17, 2019 at 6:44
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No. DMICode does not tell you about performance or provide particularly detailed info about disks. The only useful information it will provide is about the.motherboard/system model which can tell us something about the sata.controller and what types.of SSDs we can use. Smartctl and hdparm can provide the disk make and model and the iops can be lpoked up in its specs.– davidgoCommented Mar 17, 2019 at 6:45
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By latencies I assume you mean IOPS (or some derivative thereof). These vary from 10k/sec to over 1m/sec– davidgoCommented Mar 17, 2019 at 6:47
NVMe SSD latencies can be as low as 250 µs (microseconds). Much faster than the typical 2-4 ms (millisecond) latencies of HDDs that spin at 7,500 - 15,000 RPM.
0.25 ms vs. 2 ms means that an NVMe SSD will have about 1/8th the latency of a fast 15,000 RPM HDD.
However a lot of factors can affect both of these numbers. It is possible for overly-taxed SSDs to provide >1 millisecond performance, and a cache on an HDD could at times provide zippier data access to commonly-used blocks. YMMV and you should consider running your own tests for the disks you are considering deploying.
