Storage bridges have become an ubiquitous part of today's computing ecosystems. The bridges may be external or internal, with the former ones enabling a range of direct-attached storage (DAS) units. These may range from thumb drives using an UFD controller to full-blown RAID towers carrying Infiniband and Thunderbolt links. From a bus-powered DAS viewpoint, Thunderbolt has been restricted to premium devices, but the variants of USB 3.2 have emerged as mass-market high-performance alternatives. USB 3.2 Gen 2x2 enables the highest performance class (up to 20 Gbps) in USB devices without resorting to PCIe tunneling.

The last couple of years have seen many vendors introduce new products in this 20 Gbps-performance class - including portable SSDs and M.2 NVMe SSD enclosures. Host support has also started to look up. The key challenges for enclosures and portable SSDs supporting USB 3.2 Gen 2x2 include handling power consumption and managing thermals. We have been analyzing these aspects (in addition to regular performance numbers) in our reviews of the 20Gbps-performance class PSSDs and enclosures. The review below presents our evaluation report of Akasa's AK-ENU3M2-07 - a USB 3.2 Gen 2x2 enclosure for M.2 NVMe SSDs.

Introduction and Product Impressions

SSD speeds and storage capacity have improved significantly in the last decade, thanks to rapid advancements in flash technology as well as high-speed interfaces / protocols. Starting from 2.5-inch disk drives barely able to saturate the SATA III (6 Gbps) interface in the early 2010s, we now have gumstick- and palm-sized drives with PCIe 4.0 support capable of sustaining more than 7000 MBps (56 Gbps).

These SSDs have also formed the base platform for portable SSDs. Traditionally, such drives have fallen into one of the six categories below, depending on the performance profile and internal components. Recently, we have seen direct flash-to-USB controllers across all but the highest performance tier listed here.

• 2.5GBps+ class: Thunderbolt SSDs with PCIe 3.0 x4 NVMe drives
• 2GBps+ class: USB 3.2 Gen 2x2 SSDs with PCIe 3.0 x4 NVMe drives
• 1GBps+ class: USB 3.2 Gen 2 SSDs with PCIe 3.0 (x4 or x2) NVMe drives
• 500MBps+ class: USB 3.2 Gen 2 SSDs with SATA drives
• 400MBps+ class: USB 3.2 Gen 1 SSDs with SATA drives
• Sub-400MBps+ class: USB 3.2 Gen 1 flash drives with direct flash-to-USB controllers

In addition to portable SSDs, this type of segmentation is also applicable to storage enclosures. Since the mid-2010s, we have seen a regular stream of SSD enclosures hit the market, catering to 2.5", mSATA, and M.2 form-factors.

Akasa is a well-known manufacturer of thermal solutions for computing systems targeting industrial applications as well as home consumers. They have been maintaining a lineup of storage bridge products catering to different market segments since 2013. We reviewed a bunch of their M.2 SATA and NVMe enclosures last year, and came away impressed with their comprehensive lineup addressing different requirements. This review looks at the AK-ENU3M2-07, an aluminum enclosure sporting a USB 3.2 Gen 2x2 20Gbps Type-C upstream interface and a M.2 2230/42/60/80 NVMe downstream port internally.

There are currently two shipping device solutions for USB 3.2 Gen 2x2 - the Silicon Motion SM2320 used in portable SSDs like the Kingston XS2000 is a native UFD controller, while the ASMedia ASM2364 is a bridge solution more suitable for use in enclosures. The AK-ENU3M2-07 uses the latter. The enclosure itself is made of solid aluminum with ridges to aid in heat dissipation. It comes with a single Type-C to Type-C cable rated for 20Gbps operation. A single thermal gap filler is supplied in the package along with a carrying pouch and a user manual.

One of the attractive aspects of the AK-ENU3M2-07 is its tool-free nature. Accessing the internal board for SSD installation is a simple matter of loosening up the two screws on either side of the Type-C port. They are big enough to unscrew without the use of any tools. This allows the bottom panel to be slid out. The board itself is affixed to this panel, and doesn't need to be taken out for any purpose. A plastic tab to hold the M.2 SSD in place is affixed to the 2280 hole by default. Rotating this tab along the notch allows the SSD to be placed in and locked in place. Without the SSD in the picture, further rotation to make the longer arm of the tab parallel to the rear panel allows it to be completely taken out (and affixed to one of the other holes corresponding to 30mm, 40mm, or 60mm SSD lengths). After the installation of the SSD, the thermal gap filler can be placed on top. The gallery below provides pictures of the enclosure as well as the SSD installation steps.

We evaluate M.2 NVMe storage enclosures using the SK hynix Gold P31 1TB NVMe SSD. Since this SSD is used in all the relevant reviews, it makes for an apples-to-apples comparison across different products.

The table below presents a comparative view of the specifications of the different storage bridges and PSSDs presented in this review.

The key aspect that stands out is how heavy the AK-ENU3M2-07 is, compared to other enclosures using the same bridge chip. While the Akasa enclosure is 112g, the Silverstone MS12 is just 53g, and the Yottamaster HC2 is 60g. This gives the enclosure a higher thermal mass to cool down the SSD inside, and should potentially result in better thermal performance. Prior to looking at the benchmark numbers, power consumption, and thermal solution effectiveness, a description of the testbed setup and evaluation methodology is provided.

Testbed Setup and Evaluation Methodology

Direct-attached storage devices are evaluated using the Quartz Canyon NUC (essentially, the Xeon / ECC version of the Ghost Canyon NUC) configured with 2x 16GB DDR4-2667 ECC SODIMMs and a PCIe 3.0 x4 NVMe SSD - the IM2P33E8 1TB from ADATA.

The most attractive aspect of the Quartz Canyon NUC is the presence of two PCIe slots (electrically, x16 and x4) for add-in cards. In the absence of a discrete GPU - for which there is no need in a DAS testbed - both slots are available. In fact, we also added a spare SanDisk Extreme PRO M.2 NVMe SSD to the CPU direct-attached M.2 22110 slot in the baseboard in order to avoid DMI bottlenecks when evaluating Thunderbolt 3 devices. This still allows for two add-in cards operating at x8 (x16 electrical) and x4 (x4 electrical). Since the Quartz Canyon NUC doesn't have a native USB 3.2 Gen 2x2 port, Silverstone's SST-ECU06 add-in card was installed in the x4 slot. All non-Thunderbolt devices are tested using the Type-C port enabled by the SST-ECU06.

The specifications of the testbed are summarized in the table below:

The testbed hardware is only one segment of the evaluation. Over the last few years, the typical direct-attached storage workloads for memory cards have also evolved. High bit-rate 4K videos at 60fps have become quite common, and 8K videos are starting to make an appearance. Game install sizes have also grown steadily even in portable game consoles, thanks to high resolution textures and artwork. Keeping these in mind, our evaluation scheme for direct-attached storage devices involves multiple workloads which are described in detail in the corresponding sections.

• Synthetic workloads using CrystalDiskMark and ATTO
• Real-world access traces using PCMark 10's storage benchmark
• Custom robocopy workloads reflective of typical DAS usage
• Sequential write stress test

In the next section, we have an overview of the performance of the Akasa AK-ENU3M2-07 enclosure in these benchmarks. Prior to providing concluding remarks, we have some observations on the device's power consumption numbers and thermal solution also.

Performance Benchmarks

Benchmarks such as ATTO and CrystalDiskMark help provide a quick look at the performance of the direct-attached storage device. The results translate to the instantaneous performance numbers that consumers can expect for specific workloads, but do not account for changes in behavior when the unit is subject to long-term conditioning and/or thermal throttling. Yet another use of these synthetic benchmarks is the ability to gather information regarding support for specific storage device features that affect performance.

Two key aspects to look into here are whether the storage bridge is able to deliver 2GBps speeds, and whether there are any glaring red flags when compared with other similar storage bridges. In both aspects, we find the AK-ENU3M2-07 giving no cause for concern. The ATTO benchmarks provided below show similar performance for all three enclosures - the Akasa one, the Silverstone MS12, and the Yottamaster HC12. ATTO benchmarking is restricted to a single configuration in terms of queue depth, and is only representative of a small sub-set of real-world workloads. It does allow the visualization of change in transfer rates as the I/O size changes, with optimal performance being reached around 512 KB for a queue depth of 4 - this may vary based on the bridge firmware too.

Full System Drive Benchmark Bandwidth (MBps)Full System Drive Benchmark Latency (us)Full System Drive Benchmark ScoreExpand All

There is not much to choose betewen the three ASM2364 solutions in the PCMark 10 traces, as expected. From a performance viewpoint, the internal SSD matters more, as we see differences in performance between the WD_BLACK P50 and the Seagate FireCuda. Thre three enclosures come in the middle of the pack with similar scores due to the same SSD being used in them. The bridge firmware differences do not result in any major performance deviations.

Miscellaneous Aspects and Concluding Remarks

The performance of the storage bridges / drives in various real-world access traces as well as synthetic workloads was brought out in the preceding sections. We also looked at the performance consistency for these cases. Power users may also be interested in performance consistency under worst-case conditions, as well as drive power consumption. The latter is also important when used with battery powered devices such as notebooks and smartphones. Pricing is also an important aspect. We analyze each of these in detail below.

Worst-Case Performance Consistency

Flash-based storage devices tend to slow down in unpredictable ways when subject to a large number of small-sized random writes. Many benchmarks use that scheme to pre-condition devices prior to the actual testing in order to get a worst-case representative number. Fortunately, such workloads are uncommon for direct-attached storage devices, where workloads are largely sequential in nature. Use of SLC caching as well as firmware caps to prevent overheating may cause drop in write speeds when a flash-based DAS device is subject to sustained sequential writes. From a storage enclosure perspective, the key aspect to evaluate is the possibility of overheating.

Our Sequential Writes Performance Consistency Test configures the device as a raw physical disk (after deleting configured volumes). A fio workload is set up to write sequential data to the raw drive with a block size of 128K and iodepth of 32 to cover 90% of the drive capacity. The internal temperature is recorded at either end of the workload, while the instantaneous write data rate and cumulative total write data amount are recorded at 1-second intervals.

CrystalDiskMark Workloads - Power Consumption
TOP: Akasa AK-ENU3M2-07Yottamaster HC2Seagate FireCuda 1TBWD_BLACK P50 1TBSilverstone MS12Kingston XS2000 1TB	BOTTOM: Yottamaster HC2Seagate FireCuda 1TBWD_BLACK P50 1TBSilverstone MS12Kingston XS2000 1TBAkasa AK-ENU3M2-07

The Akasa AK-ENU3M2-07 and the Yottamaster HC2-C3 seem to suffer from the same firmware issues when it comes to power consumption behavior. Neither enclosure enters a lower power state successfully within the evaluated time interval, and  keeps consuming around 2.5W from the port for no reason in our setup. Contrast this with the Silverstone MS12 that enters a 0.5W power consumption state within minutes of starting to idle. From the perspective of power consumption, the AK-ENU3M2-07 can do with some improvements.

One of the idiosyncrasies we encountered with the Akasa unit was that the enclosure was becoming inaccessible after idling for some time. In order to debug this issue, we kept the power recording on for an extended duration after subjecting it to some traffic.

The problem seems to be related to the AK-ENU3M2-07's attempt to enter a low power state - after around 30 minutes of no activity, the power consumption drops from 2.6W down to around 1.1W - but, the device silently drops out (the OS doesn't realize that the device is inaccessible until some traffic is directed to the drive inside). The major issue here is that unless the enclosure is manually disconnected and reconnected, it is not possible for the storage drive to start working again. This is not an acceptable behavior for many use-cases where the SSD is expected to idle for a long duration but be ready when the user sends traffic to it. Strangely, we didn't see the problem with the Yottamaster HC12 which seems to have similar power consumption behavior in the CrystalDiskMark test. We have communicated our findings to Akasa, and hopefully, the issue can be resolved with firmware updates.

Concluding Remarks

Akasa distributes its SSD enclosures in multiple market regions. Unfortunately, they are not directly available in the North American market, with the focus in the US being more on their fanless PC cases. In the UK, the AK-ENU3M2-07 can be purchased for GBP 69, with a shipping option to the US. The best enclosure we have seen in the US market is the Silverstone MS12 (USD 70), but consumers in other regions have an alternative in the form of the Akasa unit.

Akasa's M.2 SSD enclosures lineup is quite comprehensive in terms of addressing various market requirements, and the AK-ENU3M2-07 is a worthy flagship addition. After having evaluated three different ASM2364 enclosures for 20Gbps SSDs, we have come to appreciate the different approaches taken by each vendor. Akasa's build and industrial design for the AK-ENU3M2-07 is subjectively attractive, while the thermal solution is objectively top-notch. If a power-hungry SSD is planned on being used, it gets the vote ahead of the others. The Silverstone MS12 is a more well-rounded solution. The Yottamaster HC2 uses very similar firmware to the Akasa one, but it does not support a tool-free experience. Overall, consumers will find the Akasa AK-ENU3M2-07 to be a solution satisfying most of their M.2 NVMe SSD enclosure requirements, as long as the enclosure is not left idle for too long a period when connected to the host system.