Solid-state storage is not all created equal. At the top end of the market, 3D Xpoint technologies such as Intel’s Optane and Micron’s Quant are close to RAM in performance terms with write speeds in the gigabytes-per-second range, but very expensive. At the lower end, solid-state drives might be lucky to achieve write speeds up to 500MBps, although this is still significantly faster than spinning disk HDDs.
In this article, we run through the key tiers of flash storage available, and look at how they differ in terms of performance, cost and use case.
Performance, capacity and endurance
While performance is a key differentiator, making the right solid-state storage choice is made more complex by questions of capacity and endurance. The largest commercially available SSDs now come in at 16TB, although Seagate announced it had designed a 60TB unit back in 2016.
Although SSD costs have fallen, larger-capacity drives remain expensive. Samsung’s 15.3TB TLC drive, for example, costs about £3,000. By contrast, a Seagate 16TB spinning disk HDD is under £400.
Then there is the issue of endurance. Cheaper flash storage wears out quickly. This is especially true of QLC Flash. The lifecycle for such a drive can be equivalent to filling the unit fewer than 200 times. This limits the usefulness of the technology to applications such as archiving, research and some web applications that need quick read times, and low power consumption, but not frequent writes. This, though, is being addressed by sophisticated management software and good engineering.
“Getting density, capacity and performance from high-performance disks is difficult,” says Andrew Buss, at analysts IDC. “With flash, the cost-performance ratio is almost there and performance is there.”
NVMe is an interface designed for flash media. It uses the PCIe connectivity standard and has much better I/O performance than the spinning disk-era SATA protocol which was the predominant form of SSD connectivity until recently.
NVMe is becoming mainstream as an option in storage array products and commonplace as an add-in to servers. NVMe can use any of the NAND flash types as media, as well as the likes of Optane, 3D Xpoint, and so on.
IOPS: 500,000 write, 550,000 read.
Bandwidth: 6.6GBps read, 2.3GBps write*.
Developed by Intel and Micron, 3D Xpoint comes in multiple versions, with performance close to that of volatile memory. Optane is Intel’s brand name.
The highest-performance tier is Optane DIMM in Byte Addressable Mode, which is close to RAM in speed and latency but with more capacity. But, as IDC analyst Andy Buss points out, it needs operating system and application support to work at its best.
Optane DIMM Block Addressable Mode allows applications to address storage without modification, but performance suffers and costs, like those of Byte Addressable Mode, are high. Block is suited to file-based applications, but does not deliver the same performance as Byte Addressable Mode.
Optane SSD is an alternative to flash storage, offering high and consistent performance. But it is expensive, so is best suited to applications such as high-performance storage tiers.
Micron calls its product line storage class memory, and claims that its QuantX X100 line is three times as fast as competing SSD technologies.
Z-NAND is Samsung’s Optane competitor, which achieves parity across some performance metrics. Its high performance comes from being based on SLC NAND flash. It, too, comes at a premium price.
TLC NAND flash
IOPS: Up to around 85,000, random read.
Bandwidth: Up to 2.6GBps read, 890MBps write (Toshiba XD5 M.2).
Cost: Low, but higher than QLC.
Available with either NVMe or SATA interfaces, TLC Flash is well suited to mixed read-write workloads. According to 451 Research’s Tim Stammers, most datacentre storage is now TLC flash. It stores three bits per cell, making it 50% more cost-effective than the older MLC technology and has largely replaced SLC (one bit per cell) and MLC (two bit) technologies.
Nonetheless, there are cases where SLC or even MLC are preferred, because of their higher performance. TLC focuses on capacity, not endurance, and although its read speeds are reasonable, write speeds are lower. It is, according to Jason Echols of Micron’s Storage Business Unit, a suitable technology for mainstream applications, not least because it has greater endurance than QLC.
IOPS: Up to around 70,000 random reads, 13,000 random write (Micron 5210 QLC).
Bandwidth: 540MBps read.
Cost: Less than TLC flash.
QLC is the latest flash technology, and packs in four bits per cell. This allows higher capacity, but at the expense of endurance and speed. IOPS are good in terms of read performance, but very poor for writes. For that reason, QLC storage is best suited to applications that have few write cycles, such as archiving.
In practice, there is no one single flash storage option that meets all datacentre requirements, so most IT teams use storage tiering. Matching storage to workloads optimises the mix of endurance, capacity, performance and cost.
But tiering also allows businesses to integrate other storage media, including disk and even tape. Most businesses have yet to move to a fully solid-state environment.
“Extremely few datacentres have completely eliminated disk storage,” says 451 Research’s Stammers. “The ones that have are either very deep-pocketed organisations with extreme performance needs, such as government security agencies, or some IT organisations whose storage requirements in terms of capacity are limited enough that they can justify the expense in return for simplified operations.
“So, tiering is very important in the large numbers of hybrid disk-and-flash storage systems that are still being deployed, and will continue to be deployed for many years yet. It’s also important for all-solid state hybrid systems that mix a top layer of Optane with the bulk of the data being held in flash.”
*Sources: IDC, manufacturers’ data. The author thanks IDC’s Andrew Buss for his assistance with the storage technology classification for this article.