The Sun Oracle F5100 flash-DIMM array joined Violin Memory’s flash array as the storage on the lowest latency TPC-C benchmarks (see The SSD write cliff in real life for that data).
Alert reader KD Mann pointed out that the F5100′s flash DIMMs are simply “. . . a SATA SSD without the sheet metal wrapper.” He later contended that it was the speed of redo log, not the storage devices per se, that drove latency.
A Reliable Source close to the F5100 told StorageMojo:
Yes the F5100 is just an SSD array. The DIMMs are just repackaged SSDs that were designed to optimize the amount of flash per controller based on a surface area computation from several years ago. I’d actually say that it’s much worse than a JBOD (JBOS?) since the modules aren’t hot swappable and the individual DIMMs get none of the benefits of economies of scale that typical SSDs do. The only advantage potentially is that they sized the SAS switches in the F5100 to push the IOPS limit of the DIMMs.
. . . The DIMM wasn’t a bad idea at the time, just in hindsight. The MicroSSD format didn’t exist with the MicroSATA connector (ala the MacBook air). And there was no clear standard for small format SSDs.
I’ve asked a followup question about those generously-sized SAS switches and will update the post when and if there’s an answer.
Update: Got a quick answer to the question:
The SAS switches were tuned to deliver optimal throughput and bandwidth for the devices, and to have enough capacity to max out all of them simultaneously. The germane comparison isn’t to SSDs — as I said, the miniDIMMs are just SSDs — it’s to the SAS switches in other JBODs.
The StorageMojo take
Mr. Mann’s argument that redo logs are the key TPC-C latency bottleneck is provocative. These benchmark systems are carefully configured and latency numbers are posted in the short summary documents, so it stands to reason that latency would get serious attention.
If Mr. Mann is correct, the benchmark teams are falling down on the job of showing their systems in the best possible light. How could that be?
Readers, what say you?
Courteous comments welcome, of course.