Kioxia (formerly Toshiba Memory) has launched their sixth generation enterprise SAS SSD, the PM6 series. This is the first SSD available to support the latest 24G SAS interface, doubling performance over the existing 12Gb/s SAS standard. Using 96-layer 3D TLC NAND flash memory, the PM6 offers capacities up to 30.72 TB and performance up to 4300 MB/s.

Serial-Attached SCSI (SAS) originated from the simple idea of running the enterprise-grade SCSI protocol over the Serial ATA physical layer, obsoleting parallel SCSI connections in the same way that SATA displaced parallel ATA/IDE in the consumer storage world. The first version of SAS corresponded to the second generation of SATA, with each running at 3 Gbit/s. SATA became a dead-end technology after one more speed increase to 6 Gbit/s, but SAS development has continued to higher speeds: 12Gbit SAS-3 was standardized in 2013 and "24G" SAS-4 was standardized in 2017. The "24G" is in quotes because SAS-4 actually runs at a raw rate of 22.5Gbit/s but delivers a true doubling of usable data rate by switching to lower-overhead error correction: 8b/10b encoding replaced with 128b/150b (actually 128/130 plus 20 bits of extra forward error correction), similar to how PCIe 3.0 switched from 8b/10b to 128b/130b to deliver 96% higher transfer rates with only a 60% increase in raw bit rate. Also similar to PCIe, it takes quite a while to go from release of the interface standard to availability of real products, which is why a 24G SAS SSD is only just now arriving.

Kioxia's enterprise SAS SSDs and their enterprise NVMe SSDs share the same bilingual controller ASIC and consequently the PM6 has a very similar feature set to the previously-announced CM6 PCIe 4.0 SSDs. This includes dual-port interface support for higher performance or for fault tolerance, and enough ECC and parity protection for the drive to survive the failure of two entire flash dies. The SAS-based PM6 series is limited to lower maximum throughput than the CM6, but a dual-lane 24G SAS link is still slightly faster than PCIe 3.0 x4. The higher performance enabled by 24G SAS means the PM6 can require more power than its predecessors—now up to 18W, though the drive can be configured to throttle to lower power levels ranging from 9W to 14W.

Kioxia Enterprise SSD Specifications
Model PM6 SAS CM6 NVMe
Form Factor 2.5" 15mm U.3
Interface, Protocol Dual-port 24G SAS PCIe 4.0 x4, NVMe 1.4
NAND Flash Kioxia 96L 3D TLC
Capacities (TB) 960GB,
1.92TB,
3.84TB,
7.68TB,
15.36TB,
30.72TB
800GB,
1.6TB,
3.2TB,
6.4TB,
12.8TB
400GB,
800GB,
1.6TB,
3.2TB
960GB,
1.92TB,
3.84TB,
7.68TB,
15.36TB,
30.72TB
800GB,
1.6TB,
3.2TB,
6.4TB,
12.8TB
Write Endurance 1 DWPD 3 DWPD 10 DWPD 1 DWPD 3 DWPD
Sequential Read 4.3 GB/s 6.9 GB/s

The PM6 SAS family is available in three endurance tiers: the 1 DWPD and 3 DWPD models closely correspond to CM6 NVMe models, but only the SAS product line gets a 10 DWPD tier. Maximum capacities are 30.72 TB in the 1 DWPD series, 12.8 TB in the 3 DWPD series and 3.2 TB in the 10 DWPD series. Kioxia said that 4 TB class drives are still the most popular, but this will probably be shifting toward the 8 TB models over the next year or so. The 30.72 TB models will remain more of a niche product in the near future, but they expect demand for those capacities to start picking up in 2021 or 2022. Detailed performance specifications for each model are not yet available.

SAS in general is still a growing market both in terms of number of units and bits shipped and is projected to continue growing for at least a few more years, even though NVMe is gradually taking over the enterprise SSD market. Kioxia's customer base for SAS SSDs has been divided between storage array vendors and the traditional enterprise server market. The storage array market has been quicker about migrating to NVMe so this may be the last generation of SAS SSDs to see significant adoption in that market segment. SAS will be hanging around in the enterprise server market for a lot longer, helped in part by the backwards-compatibility with SATA hard drives for cheap high-capacity storage, and the straightforward traditional RAID solutions as compared to the challenges with NVMe RAID. The server market typically doesn't make as much use of the dual-port capability of SAS drives, so the speed boost from 24G SAS will be particularly welcome there, allowing drives to now reach about 2.3GB/s each rather than about 1.1GB/s on 12Gb SAS.

The Kioxia PM6 SAS SSDs are now available for customer qualification and evaluation. The drives have already been validated with 24G SAS host controllers from both Broadcom and Microchip (Microsemi/Adaptec).

Source: Kioxia

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  • Billy Tallis - Tuesday, June 16, 2020 - link

    Packing 7mm SATA drives that densely is possible, but even 7mm U.2 drives tend to have substantially higher power draw on account of delivering much higher performance. U.2 enclosures that are denser than 15mm per drive are basically unheard of, but if one existed you would certainly hear it from a long ways off due to all the fan noise. Reply
  • Billy Tallis - Tuesday, June 16, 2020 - link

    Some NVMe drives (such as the CM6) include dual-port capability, where they can do multi-path with x2+x2 links rather than operating as a single x4 link. PCIe switches are the equivalent of SAS expanders. Broadcom's PLX and Microchip's Microsemi are the two main providers of big PCIe switches, with lane counts up to 96-100 lanes. So port counts are lower than the largest SAS expanders, but total bandwidth is way higher.

    Thunderbolt isn't the only PCIe cabling option, and isn't used in servers. External connections between a server and a PCIe JBOF are usually done with SFF-8644: https://www.serialcables.com/product-category/pcie...

    When it was first introduced, NVMe definitely was at a disadvantage to SAS in terms of enterprise-oriented features. But the NVMe ecosystem has caught up and SAS has very few remaining advantages.
    Reply
  • MenhirMike - Tuesday, June 16, 2020 - link

    Thanks for the info! I'll keep an eye out, but SFF-8644 is a good lead! (no pun intended) Reply
  • kobblestown - Wednesday, June 17, 2020 - link

    "PCIe switches are the equivalent of SAS expanders"

    In my understanding, SAS expanders implement circuit switching whereas PCIe switches are packet switched. They have similar role but fulfill it in a different way. A SAS expander will reserve a route through the topology for the entire duration of a transaction (although I think the route might be different for different transactions). So when you mostly send information downstream, say, during a write operation, the uplink channel can not be used for some independent read operation. Whereas with NVMe each packet is independent. This makes them quite different in my opinion.

    Obviously, the future belongs to NVMe. Especially over fabrics - the protocol is much less chatty.
    Reply
  • Billy Tallis - Wednesday, June 17, 2020 - link

    SAS-4 does add dynamic channel multiplexing, which somewhat relaxes the restrictions of the circuit-switched model, but I'm not sure it does anything to address the poor utilization of full-duplex bandwidth that you refer to. If only they would share the spec publicly, instead of just vaguely describing the features in press releases. Reply
  • Deicidium369 - Tuesday, June 16, 2020 - link

    2 different drives here - the SAS12 (existing) and SAS24 (new) and then the NVMe drives. Most SAS drives are mechanical - with the SAS24 - building out with SAS24 SSDs (These are NOT NVMe drives) offer a huge benefit over mech drives - from both the speed of and SSD and then the increase of bandwidth.

    So - SAS12 and SAS24 ARE NOT NVMe - with these you can use port expanders just like you can with SAS12 and SATA - they need to be SAS24 - but can use expanders.
    Reply
  • Billy Tallis - Tuesday, June 16, 2020 - link

    Clarification: SAS is fully backwards-compatible, so these new 24G SAS drives will work fine (albeit slower) when connected to 12Gb SAS hosts or expanders. Alternately, 12Gb SAS drives work fine connected to 24G SAS expanders, and the expander can talk to the host adapter at 24G speeds even if the individual drives are all slower than 24G. So 24G SAS gives expanders more headroom for aggregating bandwidth from individual drives.

    And a lot of newer SAS HBAs and RAID cards are tri-mode, supporting SATA, SAS and NVMe drives all through U.3 ports, but usually with much tighter restrictions on how many NVMe drives can be connected. So as 24G SAS rolls out, it will be increasingly common to find it deployed in a way that allows a given drive bay to hold either a SAS or NVMe SSD.
    Reply
  • Santoval - Tuesday, June 16, 2020 - link

    I had no idea a new version of SAS had been developed. I thought SAS was still (and would remain) at 12 Gbit/s. Reply
  • deil - Wednesday, June 17, 2020 - link

    Everybody crying about how QLC/TLC is bad for longevity and next second someone makes 30 TB with 1DWPD. overprovision is propably like 2x or 3x but for everyone who needs insane read speed and cappacity, without a lot of writes, this 30 TB thing with QLC (would be 40TB) that's insane. Reply
  • MenhirMike - Wednesday, June 17, 2020 - link

    QLC is basically the SSD version of Hard Drive SMR, except that it doesn't suck a terribly as SMR does. It's a nice compromise between price and capacity for read-heavy workloads - I wouldn't mind a 8-10 TB QLC SSD for a reasonable price ($600 or less), though I realize that's still somewhat off. Reply

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