Flash storage has squeezed all the traditional rotating disks into one (or just two) tiers. Also, the cost is already competitive with 15K disks (just comparing the same capacity) and it's becoming more competitive with 10K disks also.
On the other side, flash storage provides a different type of low-level technologies for the NAND chips with a decreasing cost:
|
NAND cell types |
Bit per cell |
Performance |
Type of usage |
|
Single Level Cell (SLC) |
1-bit |
Fastest both on read and write |
Used for the write-intensive flash |
|
Enterprise Multi Level Cell (eMLC) |
2-bits |
Slower (as compared to SLC) on the write operation |
Used in enterprise storage for the read-intensive flash |
|
Multi Level Cell (MLC) |
2-bits |
Average performance |
Usually for the consumer market |
|
Three Level Cell (TLC) |
3-bits |
Lower performance (but still better than rotating disks) |
Low cost and high capacity NAND flash |
And there are already new technologies, such as 3D XPoint (Intel Optane and Micron Quantx) that can provide performance much similar to DRAM, and 1,000 times faster than NAND, 10 times denser and with better endurance.
On the other hand, some applications, such as huge databases, are moving their data from disk to RAM using the in-memory paradigm, and there is also the fact that RAM memory is growing and it's becoming quite cheap. But, of course, traditional RAM is not persistent, so you still need flash or disks, although there is also the new Dual In-Line Memory Module (DIMM) based on a flash chip that works like RAM DIMMs, but provides data persistence. NVDIMM-N is another wave of new memory technologies referred to as Persistent Memory (PM), and also as Storage Class Memory (SCM).
About the connectors; for several years the SAS bus was used, usually packing the flash in a box like traditional hard disks, realizing the SAS Solid State Disk (SSD). Also, the SATA bus has been used with the Advanced Host Controller Interface (AHCI) transport protocol. The issue is, if SSD can replace an HDD with a much faster device, the entire I/O stack remains the same with an old architecture and possible bottlenecks.
NVMe or Non-Volatile Memory Host Controller Interface Specification (NVMHCI) is a new logical device interface specification for accessing non-volatile memory NVE attached through a PCI Express (PCIe) bus instead of using an SAS bus. Supported form factors include add-in PCIe cards and M.2 and U.2 interfaces. NVE is still flash memory, but now with a stack designed to reach the best performance.
The following table summarizes the different types of transport protocols:
|
Transport protocol |
Max number of queues |
Max queue depth |
|
AHCI (SATA) |
1 |
32 |
|
SAS |
1 |
254 |
|
NVMe |
65.535 |
64.000 |
NVMe reduces I/O overhead and brings various performance improvements in comparison to previous logical-device interfaces, including multiple, long command queues, and reduced latency.
As defined in http://www.esg-global.com/research:
For local storage (DAS), VMware vSphere supports SAS and SATA SSD, like other disks. The only requirement is that the controller is recognized by ESXi and that it provides the RAID function at the hardware level because ESXi does not provide any software RAID. The only exception is vSAN, but this will be explained later. NVMe or other new technologies are also supported, but you may require additional drivers.