Samsung T5 EVO Portable SSD Review: QLC Sets Sane Expectations, Insane Pricing

Miscellaneous Aspects and Concluding Remarks

The performance of the Samsung T5 EVO 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.

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.

Sequential Writes to 90% Capacity - Performance Consistency
TOP: Samsung T5 EVO 8TBSamsung T9 4TBCrucial X10 Pro 4TBCrucial X6 4TBSamsung MUF-256DA 256GBPNY EliteX-PRO 4TBSamsung T7 Shield 4TBSanDisk Extreme PRO v2 4TB	BOTTOM: Samsung T9 4TBCrucial X10 Pro 4TBCrucial X6 4TBSamsung MUF-256DA 256GBPNY EliteX-PRO 4TBSamsung T7 Shield 4TBSanDisk Extreme PRO v2 4TBSamsung T5 EVO 8TB

The T5 EVO is able to sustain maximum write speeds (around 440 MBps) for more than three hours before the direct-to-QLC writes start. In the latter state, the write speeds drop down to around 60 MBps. However, limiting the interface speed allows the controller to rapidly fold the data written earlier and free up SLC cache for the incoming data. An empty 8 TB T5 EVO allowed us to write as much as 5.2 TB of data before the direct-to-QLC segment. To be completely honest, it is difficult to find a use-case for a PSSD that involves writing of more than 5.2 TB of data in one shot.

Power Consumption

Bus-powered devices can configure themselves to operate within the power delivery constraints of the host port. While Thunderbolt ports are guaranteed to supply up to 15W for client devices, USB 2.0 ports are guaranteed to deliver only 2.5W (500mA @ 5V). In this context, it is interesting to have a fine-grained look at the power consumption profile of the various external drives. Using the ChargerLAB KM003C, the bus power consumption of the drives was tracked while processing the CrystalDiskMark workloads (separated by 5s intervals). The graphs below plot the instantaneous bus power consumption against time, while singling out the maximum and minimum power consumption numbers.

CrystalDiskMark Workloads - Power Consumption
TOP: Samsung T5 EVO 8TBSamsung T9 4TBCrucial X10 Pro 4TBCrucial X6 4TBSamsung MUF-256DA 256GBPNY EliteX-PRO 4TBSamsung T7 Shield 4TBSanDisk Extreme PRO v2 4TB	BOTTOM: Samsung T9 4TBCrucial X10 Pro 4TBCrucial X6 4TBSamsung MUF-256DA 256GBPNY EliteX-PRO 4TBSamsung T7 Shield 4TBSanDisk Extreme PRO v2 4TBSamsung T5 EVO 8TB

The T5 EVO has a peak consumption of 4.61 W. The spike in power numbers between the 700s - 835s timestamps point to rapid folding in progress, allowing the SLC cache to be regained.

Final Words

The flash market is currently experiencing a supply glut, with pricing to the advantage of the consumers. This is reflected even in SSDs such as the 870 QVO. The 8TB version of the 870 QVO is available for purchase at less than $350. The inclusion of a bridge chip and the design of a smaller enclosure with additional thermal protection can be expected to add around $50 - $75 to that price. Charitably keeping this number at $100, we would have expected the 8TB T5 EVO to retail around $450. However, Samsung is currently selling the PSSD for $650. This is absurd pricing for a QLC PSSD with speeds limited to 5 Gbps, even if we grant that no other PSSD is currently being sold in the market at that capacity point.

The 2TB and 4TB variants are being sold at $190 and $350 respectively. The 2TB number is palatable, but things start becoming awry with the 4TB version. At that capacity point, PSSDs delivering much better performance with TLC flash and faster host interfaces are available for a much cheaper price. It is not clear whether the T5 EVO presents anything unique at the 2TB and 4TB capacity points to deserve that premium.

While the value proposition is clearly not in favor of the T5 EVO at the current price points, the product definitely deserves credit for creating a new category. QLC is notorious for its slow direct write speeds, an aspect that is easy to encounter in direct-attached storage workloads. By limiting the interface to 5 Gbps (USB 3.2 Gen 1) speeds, the SSD controller is able to fold written data to the QLC segment and free up the SLC cache for the incoming data simultaneously. This allows the end user / application to enjoy the benefits of a much larger SLC cache than what is actually available.

Most PSSDs in the market employ a DRAM-less platform to bring down power consumption and cost. Interestingly, the T5 EVO adopts a board with LPDDR4 RAM for the flash translation layer. This gives the T5 EVO an edge over DRAM-less SSDs (even NVMe ones) for some workloads.

On the technical and product engineering front, the T5 EVO portable SSD is a clear winner, particularly at the 8TB capacity point. The 460 MBps cap may not matter for a wide range of use-cases that is currently being served by portable hard drives. However, the insane pricing throws a spanner in the works. Hopefully, we will see Samsung take the feedback into consideration and market the T5 EVO with a slight premium over the 870 QVO.