Why PCIe-based SSDs Are Important

There’s an old expression I like: “Different isn’t better, it’s just different.”

When it comes to SSDs based around a SATA or SAS format — that’s pretty much the case in my view. Yes there are exceptional products suited for enterprise like Pliant and STEC. And, yes — there are more conventional items for consumers like Intel and OCZ (and about 20 others).  And yes, the standard pacakge 3.5″ form factor for these devices make them suitable for shared storage as well as for integration into hetreogenous and homogenous storage environments like you might find in a typical data center.  Embracing these SSDs you will find the usual manufacturers like EMC, NetApp, SUN, and others.  Their use of SSD is evolutionary, easy to digest.

PCIe-based SSDs are very different.  For one thing, they sit on the server system bus right next to the CPU.  This is a direct attached (DAS) model that has numerous advantages for certain types of processing.  We agree that not all PCIe-based SSDs are suitable for all applications — but in terms of applications that can take advantage of bandwidth, throughput, and latency enhancements, these devices are indeed a superior architecture.

There are some challenges:

1)  Not all servers are created equal.  PCIe-based devices require strict adherance to the PCIe specifications at the server level.  Ping if you want to learn more about why this is critical.

2)  Many servers do not have enough PCIe slots configure appropriately for PCIe devices.  This is especially true when creating HIGH AVAILABILITY (or HA) environments.

3)  Only a very few servers have enough of the right type of slots to be meaningful from a value perspective.  It makes no sense to refresh a server for a PCIe-based SSD if you have to spend 2x or 3x to get the right slots, power, etc.

4)  Applications may not be optimized for SSD DAS.  No kidding.  OLTP or DBMS applications that can take the most advantage of SSD DAS are optimized for high latency disk access over networks such as NAS.  These applications are totally comfortable sending out 1000s or 10s of 1000s of transaction requests to build up a queue depth for the CPUs.  The net result of this is that the CPUs appear very busy but in fact aren’t doing very much.  These limitations are known and well defined.  Over time, application vendors such as SUN, Oracle, and Microsoft will implement fixes to optimize PCIe-based storage.

Aside from these items, there is a discussion regarding suitability of NAND flash devices in the data center as well as the MLC/SLC issue.  I’ll tackle those in another post.  In my veiw, MySpace and Wine.com are leading the way — and there are many others who have not come forward publicly preferring to keep the ROI and GREEN advantages all to themselves.

The latest announcements from Fusion-io, Texas Memory Systems, Micron and others point out these differences.  FULL DISCLOSURE:  I am a former employe of Fusion-io.

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