Modern magnetic recording media, the building block of HDDs, has higher durability than most modern NAND flash memory used in SSDs. However, controversial workload ratings imposed by HDD makers to diversify their product portfolio make the latest NAS-bound hard drives less reliable than even cheap SSDs, much less the best SSDs on the market.
There are several breeds of HDDs available today: enterprise-grade drives rated for a 550TB workload per year, enterprise NAS-grade drives rated for a 300TB workload per year, and desktop PC drives rated for 180TB per year or so. Solid-state drives do not have per-year workload ratings but feature terabytes to be written (TBW) and drive writes per day (DWPD) endurance ratings that you can use to calculate the total endurance.
|Annual Workload Rating||Terabytes Written|
|WD Gold 20TB HDD||550 TB||2,750 TB|
|WD Red Pro 20TB HDD||300 TB||1,500 TB|
|WD_Black SN850 2TB SSD||240 TB||1,200 TB|
|WD Red SA500 4TB SSD||500 TB||2,500TB|
|Ultrastar DC SN840 6.4TB||7,000 TB||35,000 TB|
ServeTheHome recently compared the latest Western Digital WD Red Pro 20TB (opens in new tab) NAS hard drive's 300TB annual workload (which includes reads and writes) to the endurance ratings for modern SSDs. The WD Red Pro comes with a five-year limited warranty, so it is designed to withstand a workload of 1,500TB (including reads and writes). Competing IronWolf Pro HDDs from Seagate have a similar workload rating, and the same applies to products from Toshiba.
In contrast, the endurance of Western Digital's own 2TB WD_Black SN850 SSD (opens in new tab) is 1,200TB over five years, whereas a 4TB WD Red SA500 NAS SATA SSD (opens in new tab) is rated for 2,500TB over five years, which is higher than the Red Pro 20TB. Both of these WD_Black and WD Red SSDs cater to mixed and read-intensive workloads, so we aren't even talking about enterprise-grade SSDs for write-intensive workloads. For example, Western Digital's Ultrastar DC SN840 (opens in new tab) 6.4TB drive is suitable for 35,000TB written over five years.
Needless to say, if we compare endurance ratings of SSDs and HDDs in terms of endurance per terabyte of capacity, SSDs will win again. Does this mean that SSDs are now more durable than HDDs in terms of the possible amount of data that could be written? Well, yes and no.
The actual HDD platters are still considerably more reliable than NAND flash media simply because modern 3D TLC NAND is rated for about 10,000 program/erase cycles. SSD makers have to use very sophisticated controllers supporting these tricky coding and error correction algorithms to improve endurance at the device level.
By contrast, magnetic media (the coating on the platters) can be re-written practically without any limits (at least with no limits worth mentioning). But modern hard drives use high-precision mechanics and motors that spin at 5,000 – 7,200 RPM. While they are reliable, they are fragile and sometimes fail, which is why hard drive makers introduced workload ratings to their products several years ago.
HDDs and SSDs in data centers are used in tiered storage subsystems and in RAID mode to store data reliably. In any case, we cannot say that one type of storage device is more reliable than another — they all can fail and require safety measures to store data reliably.
But judging only by workload and TBW ratings, SSDs are at least as reliable as NAS-oriented WD Red Pro HDDs. Of course, datacenter-bound hard drives feature higher-endurance ratings and are for essentially 2,750 TBW over five years, but datacenter SSDs for write-intensive workloads can beat even them.
While HDDs and SSDs have pros and cons, it is essential not to rely on a single device to store your data. While workload, TBW, and DWPD ratings are crucial, you should not rely on just one metric for reliable storage, as you need more than one device to ensure the safety of your data.