OWC Mercury Pro U.2 Dual
It seems the stars aren’t yet aligned for U.2 to succeed: prices need to drop and performance must improve
PRICE Diskless, $300 from megamac.com
The problem with M.2 NVMe
SSDs is simple: size. The form factor’s minimal surface area limits the number of NAND modules that can be mounted on it, currently topping out at 8TB. U.2 is a relatively new specification that provides more space for modules and, therefore, much greater capacities.
Always at the pointy end of storage developments, OWC has sent us a product designed to exploit U.2 technology for rapid external storage. This Thunderbolt 3 drive enclosure can accept two U.2 drives and use them in a RAID configuration. Or, via an adapter, up to eight M.2 SSDs.
As we went to press, OWC only had US prices, but its UK release is set for mid-January so you won’t have long to wait to find out exactly how much you’ll be paying. It will be offered either as a bare drive or with preinstalled U.2 drives, and OWC intends to sell it with its own SoftRAID software in the future. As a guide, a bare Mercury Pro Dual unit costs
$300, rising to $380 with a SoftRAID licence. We expect that to translate to around £300 and £380 in the UK.
Mercury Pro designs with U.2 storage include 2TB (2 x 1TB), 4TB (2 x 2TB), 8TB (2 x 4TB) and 16TB (2 x 8TB) versions, and they cost around $500, $680, $950 and $2,500 respectively. Promised soon are the SoftRAID NVMe M.2 derivatives. These will range from 8TB (8 x 1TB) to 64TB (8 x 8TB), with prices between $2,199 and $12,799.
With or without U.2
The Mercury Pro’s design exudes a minimalist aesthetic, with a pair of activity LEDs and two grille vents to break the solid mass of black aluminium tubing. There’s no power switch: once the enclosure is connected to the external PSU, it’s on.
Two Thunderbolt 3 ports sit on the back, with one cable in the box. The inclusion of two ports permits the daisy-chaining of Thunderbolt devices, allowing a single port on the computer to be distributed to more than one device.
Access to the internal bays is via two screws on the back. Once removed, the internal cradle on which the drives are mounted slides out, allowing drives to be installed or replaced. You aren’t forced to install both drives, but they must be of the U.2 specification to connect inside this enclosure.
OWC makes a single M.2 tray with a U.2 connector, the U2 ShuttleOne. For those that invest in the M.2 version of the Mercury Pro, it comes with a different tray that can take four M.2 drives for each U.2 connector.
The obvious choice would be to buy the barebones system with the
M.2 four-drive tray for maximum flexibility, but that isn’t an option that OWC currently offers. I can only hope it removes this restriction.
Mysterious ways
On my test PC, I used Windows drive management to format both supplied drives to NTFS for benchmarking. My system included two Western Digital SN630 U.2 960GB units; once formatted, they appeared as any other external drives might.
I told Windows to stripe the two drives using the software RAID functionality built into the OS, but that failed. It could be that the drives had been partitioned in a specific way that wouldn’t allow for the conversion to a dynamic structure needed for striping by the OS. That said, the software RAID in Windows doesn’t actually deliver the performance impact you might expect, so this wasn’t a huge disappointment.
Fortunately, OWC has its own RAID software that’s designed to work with all its external multi-drive solutions, called SoftRAID. That OWC doesn’t include any version of this with the hardware as standard is very disappointing. Without it, the two drives can’t be striped or mirrored at all.
To achieve those configurations requires SoftRAID Lite, priced at $50. This tool supports external drives of any brand, not only those made by OWC. However, the version that OWC bundles with the SoftRAID-enhanced
Mercury Pro packages (for which it charges that extra $80) is SoftRAID XT, and that doesn’t support thirdparty hardware.
SoftRAID XT is a step up from the Lite version, in that it comes with technical support and the ability to configure RAID0, 1, 4, 5 and 1+0. However, RAID4, 5 and 1+0 are currently only options on a Mac, so it’s a pointless investment for PC owners.
SoftRAID XT costs $120 on its own, while the top rung of this software ladder is SoftRAID Pro. It costs $250, for which you get the ability to work with any brand of drives, and to spread volumes over multiple drives. If you’re unsure which of these you want, OWC offers fully functional 14-day trials on all flavours to help you make that assessment.
OWC promotes SoftRAID as a much less expensive and more flexible approach to RAID, and both of those points are valid. But hardware RAID isn’t purely about being costly to use. It also offers the best levels of performance, and because the controller is independent of the computer, a system crash or errant application shouldn’t corrupt the volume. Some RAID controllers can hold unwritten data if the power is interrupted, and this isn’t a feature that software RAID can realistically deliver.
I should also mention that using striping, as the Mercury Pro is designed
“To put it bluntly, for the performance that’s available and the high cost, these numbers don’t add up”
for, instantly doubles the statistical chance of losing the volume through hardware failure. What’s more, having that volume over multiple devices introduces additional points of potential failure. Yes, it will be faster, but there is a trade-off in resilience or capacity.
Two disks beat as one
There are also performance issues with the Mercury Pro U.2 Dual, and these are all to do with the bandwidth offered by Thunderbolt 3 and 4. Using all the bandwidth of a Thunderbolt channel offers a theoretical cap of around 32.4Gbits/sec, or 4.05GB/sec. But I’ve never seen an external drive of any technology with a speed that exceeded 2.8GB/sec, because the reality of Thunderbolt is less than the theoretical bandwidth.
The two 960GB Western Digital SN630 NVMe U.2 drives in my test system aren’t quick by NVMe standards: each is theoretically capable of sequentially reading at 2.7GB/sec and writing at 930MB/sec. For reading alone, then, a single drive can take all the available bandwidth to achieve its potential, but that doesn’t happen. Instead, Thunderbolt limits each drive to about half of the total available bandwidth, unless the drive is chained, and then it would be even less.
In my tests, the sequential read speed of a single drive without SoftRAID was 1,630MB/sec, and writing was 1,039MB/sec. That’s about a quarter of the speed you might see from an internally mounted PCI-E 4 M.2 NVMe SSD.
Using SoftRAID XT, I then explored what performance gains can be achieved using both RAID0 (stripe) and RAID1 (mirror). The RAID0 striped option boosted read speed to 2,756MB/sec and write performance to 1,915MB/sec. Read performance is very close to the practical limit of Thunderbolt bandwidth, but well inside the performance envelope for the SN630. The other advantage of stripe mode is the combined drive is double the size, so 1,788GB in this instance.
Mirroring using RAID1 has the resilience to handle a drive failure, but it also halves the capacity. And, in my testing, speeds dropped to the equivalent of a single drive.
However, it’s worth considering that the SN630 was designed as a data centre SSD and not specifically for external drive storage devices, and connecting it via Thunderbolt negates much of its potential performance.
Then consider the option of using up to eight M.2 NVMe drives, which takes even greater performance potential and tries to feed it through a relatively narrow Thunderbolt channel. Because Thunderbolt is the limiting speed factor, it would be possible to achieve the same speed to and from the PC by combining SATA SSDs, at a fraction of the cost.
Sweetest thing?
U.2 technology has been promoted as a magic wand that addresses the size limitations of M.2 drives. But the released drives so far are only twice the size of M.2 sticks. I presume that they’ll get bigger, but maybe M.2 capacity will also be impacted by the same chip developments?
The other problem with the U.2 concept is the price, which appears artificially inflated considering they don’t offer a performance advantage over M.2 since they use the same
NVMe interface.
Our review Mercury Pro used 960GB WD SN630 U.2 drives that cost £307 each. That’s almost triple the typical price of an M.2 NVMe drive for half the performance levels. The 8TB capacity WD SN630 costs £1,356 on eBay, which is over £100 more than the 8TB M.2 Rocket Q drive made by Sabrent. Other suppliers have more affordable U.2 options, but few are as cheap as M.2 NVMe drives.
OWC has done what it can to make the drives bundled with the Mercury Pro cheaper than retail. But for those buying a bare unit or planning to upgrade, this could be an expensive exercise. To put it bluntly, for the performance that’s available and the high cost, these numbers don’t add up.
Maybe when U.2 becomes more established, the reduced cost per terabyte and improved capacity sizes will make the Mercury Pro U.2 a more attractive option. But currently it doesn’t offer cost advantages over an internally mounted M.2 NVMe drive; it’s merely easier to move around.
The largest capacity U.2 drives we’ve seen are the 15.36TB Samsung PM1733 and Intel SSD D5 P4326, which are twice the size of M.2 NVMe at more than double the price. For the Mercury Pro to become more desirable, U.2 drives need to be 32TB or bigger and cost the same as M.2 drives of equivalent capacity. And when Thunderbolt 5 arrives, it needs to provide enough bandwidth to ensure that having NVMe drives on this bus doesn’t massively undermine their performance. Unfortunately, none of that is likely to happen any time soon.
SPECIFICATIONS
2-bay external disk enclosure 2 x USB-C Thunderbolt 3 (15W power delivery per port) 2 x 2.5in or 3.5in U.2 SSDs supports 8 x NVMe M.2 SSDs via adapter supports RAID0, RAID1, JBOD and SPAN up to 2,800MB/sec reads 85 x 239 x 147mm (WDH) 1yr limited warranty