There is a curious quiet from the enterprise storage community on form factor and interface direction for solid state storage, be it NAND Flash, Intel 3D XPoint or other. On the client-side, personal computing, both desktop and mobile, show clear direction in favoring both the M.2 form factor and PCI-E as the preferred interface for SSD storage. There is a backward compatibility option in M.2 to work with either SATA or PCI-E interface, but I do not think this will be widely used. SATA or hard disks will not go away, only the primary SSD is M.2 form factor, PCI-E interface and NVMe host protocol.
On the enterprise side, there is great deal of deployed infrastructure built around the SAS interface (a super-set of SATA), and the small form factor (SFF) for 2.5in HDD at 15mm height or thickness. The bean counter types would like to wish that SSD (NAND flash for those who do not like the acronym SSD) would use existing infrastructure and not just as an interim measure. They are probably still unhappy that Fiber Channel on the back-end had to be abandoned several years ago, being not competitive and a cost burden relative to SAS.
Preserving the value of investment in existing infrastructure is important because people are unhappy when equipment purchased at painfully high cost becomes obsolete. Of course, enterprise storage is only extremely expensive because storage vendors invented justifications for selling inexpensive components at very high markup.
There is also a failure to consider that hardware has an effective depreciation of 30-40% per year due to the pace of progress, which renders the term investment in hardware foolish, or if I were less polite, then completely stupid. So ultimately this is circular logic based on an untenable premise.
That said, it would be possible to build a viable enterprise storage system around either the PCI-E or SAS interface, because both support multiplexing lanes, and there are switch chips for both PCI-E and SAS interfaces. The reason PCs are moving from SATA to PCI-E is that the NAND interface bandwidth is advancing at a faster pace than any single lane connection can support and SATA does not support multiplexing. (In websites catering to desktop hardware, some say that PCI-E is superior to SATA. This is rubbish by writers with poor technical knowledge. The only important fact is whether the interface supports multiplexing.)
The reason existing enterprise infrastructure should be abandoned is not because of any deficiency in SAS, but rather that it is built around four lane (x4) uplink and downlink ports. SAS at 12 Gbits/s would only support 4.4GB/s net bandwidth. This might seem to be high because enterprise storage vendors sell crappy systems with pathetic bandwidth capability. The other reason is that most existing infrastructure are either the 24-25 bay SFF in 2U or 15-bay LFF in 3U 19-inch wide rack mount enclosures designed for hard disks. Both the number of bays and physical volume are completely wrong for current generation SSDs going forward.
My opinion is that the correct uplink and downlink for solid state (be it NAND flash, Intel 3D XPoint or other) storage enclosures (not individual devices) should be 16 lanes wide or x16. Both PCI-E and SAS have adequate bandwidth and protocols. For PCI-E gen 3 at 8Gbit/s per lane, this would support a net bandwidth of 12.8GB/s. The existing x4 SAS is just too low for an SSD (formerly disk) array enclosure.
The core of this argument is based around the PC standard of a Flash controller with 8-channels on the NAND side, and PCI-E on the uplink side. Today the NAND interface is 333MB/s, so an 8-channel controller could support 2.6GB/s. There may have been some thought that the upstream side should be PCI-E gen 3 with 2 lanes (x2, capable of 1.6GB/s), as it is common to have excess bandwidth capability on the down stream side. But in PC world, single device benchmark performance is important, so the trend seems to be PCI-E x4 on the controller, with the option to connect only x2 (or even x1?).
In the time of hard disks, client side PCs used 7200 RPM HDDs or less, for lower cost and higher capacity. Enterprise storage was primarily 10K or 15K RPM for greater IOPS performance, although 7200 RPM was adopted for tier 2 storage. (Storage capacity should have been too cheap to meter even for 10K HDDs, but because vendors sold a ridiculously high prices, this created a demand for 7.2K in enterprise storage systems.)
In the first phase of SSD adoption, enterprise systems preferred single level cell (SLC) NAND with greater write endurance while client side was mostly 2-bit MLC and later some low cost devices being 3-bit TLC. Today NAND flash technology is sufficiently mature that MLC has sufficient write endurance for many enterprise needs. Fundamentally, the performance oriented PC and enterprise could use the same SSD, just with different over-provisioning and other firmware settings. It would be foolish for enterprise systems not to leverage components developed for client side systems, given the huge volume and low cost structure.
While the standard desktop SSD element is M.2 form factor with an 8-channel controller and capable of x4 on the upstream side, the enterprise strategy should be to connect x2 on the upstream side. In enterprise, it is the performance of the complete array of storage elements that is important, not the single component. The standard storage array enclosure should probably have 16 bays, each connected x2 to the PCI-E switch, and x16 for each of the uplink port and downlink expansion port. The PCI-E switch would have 64 ports, 16 for uplink, 16 for downlink expansion, and 16 x2 for the M.2 SSDs. The enclosure should work with either 1 or 2 controllers. Existing DAEs have a single (x1) SAS connection to each bay.
The physical volume for 16 M.2 devices would occupy only one-quarter of 1U rack. Existing enterprise storage infrastructure is x4 uplink/downlink ports, 2U full rack with 24-25 bays connected x1. This wrong for SSDs at on multiple points. Uplink and down link ports should be x16. The volume of the enclosure should be shrunk by a factor of 8. Connections to each bay should be x2, but 16 bays connected at x1 is weakly viable. Given that existing infrastructure is unsuitable for SSDs going forward, there is no excuse to not adopt the client-side components with M.2 form factor and PCI-E in a new properly designed infrastructure.
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Good question. Sorry about the long winded answer, but life is complicated. I do agree with shared storage concept in HDD days, having a common pool of HDD so that each host can access the aggregate IOPS capability when needed. This and managing the complex storage system alone would have justified a good profit margin. But storage felt the need to justify extraordinary margins, hence started to invent reasons, which led to doctrine based on the invented justifications. Any time happens, it is a fuck-up of the first magnitude. And storage vendors do not seem to understand what bandwidth is, or about log write latency.
Next, blade systems are non-starter in databases because it gives up DIMM slots and PCI-E slots. So we should stick with rack systems with the full boat of DIMM and PCI-E slots. Today a 4TB PCI-E is do-able. Whats missing is some way to match PCI-E SSDs to the available PCI-E lanes. System vendors have a mix of PCI-E slots, including several x16. Only workstations and HPC have x16 cards, servers do not. So we want to connect four PCI-E x4 SSDs to x16 slots. HP workstations have some card for this, but we need a server version. I can see a 4-socket server with 64 - 128 PCI-E lanes dedicated to storage, that is 16-32 x4 PCI-E SSDs, so 64-128TB in one system. All this can be internal, SSDs do take much space, aren't too heavy and don't consume much power.
Storage vendors still want to sell horribly expensive AFA storage with features that we don't need, and cannot deliver anywhere close to the bandwidth that is possible. So it really is a fight between the DBA for cheap SSD at high bandwidth, and the SAN admin who wants to spend a shit load of money, have control over all storage, make you fill out forms to justify why you need each precious GB of space, all so he/she can deny you request as not sufficient in his/her judgment.
Given that the NVMe controllers seem to be PCI-E x4, perhaps the strategy should be x16 uplink and downlink, with 8 x4 bays. There is still a 2:1 mismatch between downstream and upstream. The point being bandwidth max is reached with 4 devices, but there is space for 4 more. Above, I suggested 16 x2 bays.