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SSDs choked by crummy disk interfaces

Gotta be PCIe and not SAS or SATA

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A flash device that can put out 100,000 IOPS shouldn't be crippled by a disk interface geared to dealing with the 200 or so IOPS delivered by individual slow hard disk drives.

Disk drives suffer from the wait before the read head is positioned over the target track; 11msecs for a random read and 13msecs for a random write on Seagate's 750GB Momentus. Solid state drives (SSDS) do not suffer from the lag, and PCIe flash cards from vendors such as Fusion-io have showed how fast NAND storage can be when directly connected to servers, meaning 350,000 and more IOPS from its ioDrive 2 products.

Generation 3 PCIe delivers 1GB/sec per lane, with a 4-lane (x4) gen 3 PCIe interface shipping 4GB/sec.

You cannot hook an SSD directly to such a PCIe bus with any standard interface.

You can hook up virtually any disk drive to an external USB interface or an internal SAS otr ATA one and the host computer's O/S will have standard drivers that can deal with it. Ditto for an SSD using these interfaces, but the SSD is sluggardly. To operate at full speed and so deliver data fast and help keep a multi-core CPU busy, it needs an interface to a server's PCIe bus that is direct and not mediated through a disk drive gateway.

What could go wrong with this rosy outlook? Plenty; this is IT. There is, of course, a competing standards initiative called SCSI Express.

If you could hook an SSD directly to the PCIe bus you could dispense with an intervening HBA that requires power, and slows down the SSD through a few microseconds added latency and a hard disk drive-connectivity based design.

There are two efforts to produce standards for this interface: the NVMe and the SCSI Express initiatives.


NVMe, standing for Non-Volatile Memory express, is a standard-based initiative by some 80 companies to develop a common interface. An NVMHCI (Non-Volatile Memory Host Controller Interface) work group is directed by a multi-member Promoter Group of companies - formed in June 2011 - which includes Cisco, Dell, EMC, IDT, Intel, NetApp, and Oracle. Permanent seats in this group are held by these seven vendors, with six other seats held by elected representatives from amongst the other work group member companies.

It appears that HP is not an NVMe member, and most if not all NVMe supporters are not SCSI Express supporters.

The work group released a v1.0 specification in March this years, and details can be obtained at the NVM Express website.

A white paper on that site says:

The standard includes the register programming interface, command set, and feature set definition. This enables standard drivers to be written for each OS and enables interoperability between implementations that shortens OEM qualification cycles. ...

The interface provides an optimised command issue and completion path. It includes support for parallel operation by supporting up to 64K command queues within an I/O Queue. Additionally, support has been added for many Enterprise capabilities like end-to-end data protection (compatible with T10 DIF and DIX standards), enhanced error reporting, and virtualisation.

The standard has recommendations for client and enterprise systems, which is useful as it means it will embrace the spectrum from notebook to enterprise server. The specification can support up to 64,000 I/O queues with up to 64,000 commands per queue. It's multi-core CPU in scope and each processor core can implement its own queue. There will also be a means of supporting legacy interfaces, meaning SAS and SATA, somehow.

A blog on the NVMe website discusses how the ideal is to have a SSD with a flash controller chip, a system-on-chip (SoC) that includes the NVMe functionality.

What looks likely to happen is that, with comparatively broad support across the industry, SoC suppliers will deliver NVMe SoCS, O/S suppliers will deliver drivers for NVMe-compliant SSDs devices, and then server, desktop and notebook suppliers will deliver systems with NVMe-connected flash storage, possibly in 2013.

What could go wrong with this rosy outlook?

Plenty; this is IT. There is, of course, a competing standards initiative called SCSI Express.

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