SCSI Trade Association (STA) Adds New Specs
September 11, 2013, Intel Developers Forum, San Francisco—The SCSI Trade Association is updating their specifications to match the performance of SSDs in the storage systems.
The multiple layers of storage can all communicate over a single protocol over the various transport layers. The SAS spec now supports 12Gbps and 1.2M IOPS to allow for the inclusion of SSDs in the various layers. Products are already available for the 12Gb data rates, and s are interoperable across all of the SCSI member products.
A new addition is the SCSI Express which allows SCSI functions over the PCIe transport layer. The SCSI express on the PCIe bus uses the standard transport layer to support SSD in RAID configurations as well as bridging to increase performance and offer greater standardization.
Some products and components in the SCSI lineup include basic SCSI, SCSI over PCIe, and now a new SCSI Express bay for multiple drives in an enclosure. The new bay standard offers up to 25W of power on the PCIe bus.
One new function is a copy using tokens. This function enables disk-to-disk copies without direct CPU involvement, a virtual disk-to-disk DMA. The capability is in the new SCSI Express, and if the drives are on the same PCIe fabric there is no network traffic for the transfer. Other functions include atomic operations like single write, or single read. Hinting is for trimming SSDs and is equivalent to unmap to reduce overhead.
A set of features that define the minimum feature set is in committee. The new functions will allow the spec to support another doubling of data rates to 24Gbps in ’16, with products shipping in late ’17. Hot swap is not supported on PCIe, but an unexpected swap functions is in development, and likely to be released in the generation 2 PCIe spec. the basic protocol is already available.
The 24Gb STA standard will be back compatible with 3, 6, and 12Gb specifications through a negotiation process on a frame-by-frame basis. The existing cables and connectors have been checked for operation at 24Gb and will work, with the same loss budget available. The encoding will be changed to accommodate the higher data rates to a 128/130 plus some forward error correction. The 24G is feasible on a 5-6M cable or a 25-inch backplane
