Lately a number of PCIe offerings have hit the SSD market. The SSD Guy breaks them into two camps: One-Hop SSDs, in which the commands are translated directly from PCIe to the NAND flash without going through an intermediary protocol, and Two-Hop SSDs, which use off-the-shelf HBAs and SATA SSD controllers to move commands first from PCIe to SATA then from SATA to NAND. There are aslo versions that go through SAS: PCIe to SAS, then SAS to NAND.
The SSD Guy figured that Easter would be a good time to talk about these since everyone already has the Easter Bunny hopping through their minds!
It’s not hard to understand why the one-hop approach might be faster, but the two-hop approach offers pretty good performance for a very reasonable development cost, and that translates to a lower price. While a one-hop controller requires a dedicated design effort, a two-hop PCIe SSD can be manufactured by almost anyone by teaming an off-the-shelf SSD controller with an off-the-shelf HBA or RAID controller, using firmware supplied by the manufacturers of the controllers.
Here is a list of the companies that I know of that are involved in either camp as of the writing of this post:
|Texas Memory Systems (IBM)||Intel|
There will be more, and I have doubtlessly missed a few. I will update this post as I am made aware of them. I ask PCIe SSD makers not to be shy, but to contact me directly if I have missed them.
Objective Analysis covers the market for PCIe SSDs in its report The Enterprise SSD: Technologies and Markets, which can be purchased for immediate download from our website.
2 thoughts on “One-Hop vs. Two-Hop PCIe SSDs”
Ahhh! Great write up on one vs two hops. I wondered what the difference is between these PCIe SSDs.
add Oracle to the two hop list, their PCIe drives are SAS controllers with NAND on it. Fusion-io has really lead the charge on the NAND as another memory tier, rather than just a hard disk replacement. lol@ OCZ for the original Revodrive. I’m not sure what they were thinking designing it around a cheap Sil3124 (the $10 ebay RAID cards). the controller wasn’t even native PCIe, the RAID controller was natively PCI-X bridged with a bridge to the PCIe bus. you should make a three-hop category for OCZ.
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