SSD = Solid State Drive; HDD = Hard Disk Drive.
The Summary:
For daily operations on a desktop or laptop computer, SSDs are better (read: faster, quieter, more energy efficient, potentially more reliable) than HDDs. However, HDDs cost considerably (6.5 times) less than SSDs. Thus, HDDs are still viable for backup storage, and should be able to last at least five years. At the end of that time, it may be appropriate to go over to SSDs, if prices continue to fall.
The Details:
This weblog post is being written as I contemplate buying two more external hard disk drives (HDDs), one white and one blue. These will be yet more supplementary backup disks to duplicate storage on our Network Attached Storage (NAS) server, Mothership, which features 4 x 10 GB Toshiba N300 internal 3.5″ hard drives rotating at 7200 RPM. These were purchased 2018-12-27. While the NAS has its own backup allowing up to two HDDs to fail simultaneously, a fire or other catastrophe would void this backup. Thus, external HDDs are used to store data at a secret, yet secure location away from our residence.
The last time external hard disks were purchased was 2018-09-04. These were Western Digital (WD) My Passport 4TB units, 2.5″ form factor, rotating at 5 400 RPM, with a USB 3.0 contact. One was red (costing NOK 1 228) and the other was yellow (at NOK 1 205). However, we have nine other 2 – 4TB units, some dating from 2012-11-15. Before this we had at least 4 units with storage of 230 GB – 1 TB, dating to 2007-09-01. (We are missing emails before 2006, so this is uncertain territory, although if this information were required, we have paper copies of receipts that date back to 1980).
The price of new WD My Passport HDD 4TB units has fallen to NOK 1 143. New WD My Passport Solid State Drive (SSD) units cost NOK 2 152 for 1TB, or NOK 3 711 for 2TB. That is a TB price of about NOK 1 855, in contrast to about NOK 286 for a HDD. This makes SSDs about 6.5 times more expensive than HDDs.
I am expecting to replace the disks in the NAS, as well as on the external drives, about once every five years. Depending on how fast the price of SSDs sink in relation to HDDs, these proposed external HDDs could be the last ones purchased.
As the price differential narrows, other disk characteristics become more important. Read/write speed is especially important for operational (as distinct to backup) drives. Typically, a 7200 RPM HDD delivers an effective read/write speed of 80-160MB/s, while an SSD will deliver from 200 MB/s to 550 MB/s. Here the SSD is the clear winner, by a factor of about three.
Both SSD drives and HDD’s have their advantages and disadvantages when it comes to life span.
While SSDs have no moving parts, they don’t necessarily last longer. Most SSD manufacturers use non-volatile NAND flash memory in the construction of their SSDs. These are cheaper than comparable DRAM units, and retain data even in the absence of electrical power. However, NAND cells degrade with every write (referred to as program, in technical circles). An SSD exposed to fewer writes will last longer than an SSD with more. If a specific block is written to and erased repeatedly, that block would wear out before other blocks used less extensively, prematurely ending the SSD’s life. For this reason, SSD controllers use wear levelling to distribute writes as evenly as possible. This fact was brought home yesterday, with an attempt to install Linux Mint from a memory stick on a new laptop. It turned out that the some areas of the memory stick were worn out, and the devise could not be read as a boot drive. Almost our entire collection of memory sticks will be reformatted, and then recycled, a polite term for trashed!
Flash memory was invented in 1980, and was commercialized by Toshiba in 1987. SanDisk (then SunDisk) patented a flash-memory based SSD in 1989, and started shipping products in 1991. SSDs come in several different varieties, with Triple Level Cells (TLC) = 3 bit cells offering 8 states, and between 500 and 2 000 program/ erase (PE) cycles, currently, the most common variety. Quad Level Cells (QLC) = 4 bit cells offering 16 states, with between 300 and 1 000 PE cycles, are starting to come onto the market. However, there are also Single Level Cells (SLC) = 1 bit cells offering 2 states, with up to 100 000 PE cycles and Multi-Level Cells (MLC) = two level cells with 2 bits, offering 4 states, and up to 3 000 PE cycles. More bits/cell results in reduced speed and durability, but larger storage capacity.
QLC vs TLC Comparisons:
Samsung 860 EVO SSDs use TLCs while Samsung 860 QVO SSDs use QLCs. The 1TB price is NOK 1 645 (EVO) vs 1 253 (QVO), almost a 25% price discount. The EVO offers a 5-year or 600 TBs written (TBW) limited warranty, vs the QVO’s offers 3-years or 360 TBW.
With real-world durability of the QVO at only 60% of the EVO, the EVO offers greater value for money.
It should also be pointed out that both the EVO and QVO have a 42GB cache that allow for exceptionally fast writes up to that limit, but slow down considerably once that limit has been reached.
In contrast to SSDs, HDDs rely on moving parts for the drive to function. Moving parts include one or more platters, a spindle, an read/ write head, an actuator arm, an actuator axis and an actuator. Because of this, an SSD is probably more reliable than an HDD. Yet, HDD data recovery is better, if it is ever needed. Several different data recovery technologies are available.
The Conclusion:
The upcoming purchases of two My Passport 4TB external HDDs may be my last, before going over to SSDs for backup purposes, both on internal as well as external drives. Much will depend on the relative cost of 10TB SSDs vs HDDs in 2023, when it will be time to replace the Toshiba N300 10TB HDDs.
For further information on EVOs and QVOs see Explaining Computers: QLC vs TLC SSDs; Samsung QVO and EVO.