Clariion CX3-40 Mixing SATA and FC shelves on the same Bus

Whoa there Kelleg...as the original author of both the Fundamentals and Best practices guides, and who shepherded them along for 5 years, killing many myths along the way...I must find myself again killing myths. In this case:

"Writing to a 7200RPM disk will take longer than writing to a 15K disk. So the slower disk will hold the bus longer."

The latter statement is not true. I can categorically state that we have never made that statement in either guide. But I understand that the misconception is widely held.

Here's the lowdown:

There have been times when we suggested using a different bus for certain drives - when they were at a lower interface rating, 2 Gb/s interface vs. 4 Gb/s. If you had the original 10K rpm drives for the CX3 series, for example, they were 2 Gb/s. You would not want to put them on a bus with 4 Gb drives as then the bus arbitrates down to 2 Gb/s. But our SATA drives are 4 Gb/s -- they will work on the bus just as well as any FC 4 Gb/s drive. So, why do folks think they 'slow down' the bus.

The confusion stems I think from 2 things:

1) statements that guided the user to separate high-bandwidth disk traffic (which is often but not always on SATA) from random. You don't want your backup to disk traffic to soak up all the bus bandwidth, affecting your online apps like Exchange. This has NOTHING to do with the disk transfer rate - it is about the workload you are asking the disks and the bus to absorb. If your BU2D is close to or more than 300 MB/s, you are using up most of one bus's bandwidth. You should put your exchange drives on another bus.

2) the misunderstanding that SATA drives somehow 'slow down' the bus. Nothing could be further from the truth. There are 3 factors at work here:

2-a) The disk transfer rate is immaterial - the transfer onto the bus is made from buffers in the HADA electronics, long (in computer terms) after the transfer from the platter is made, and it is controlled by the interface electronics.

2-b) The arbitration for the bus is handled by the interface electronics which, in the SATA II product we have shipped for over a year now, is 4 Gb/s capable. So, no slowdown from that.

2-c) The transfer PATTERN is really controlled by the SP. Those manufacturer specs for MB/s transfer rates are really just pie-in-the-sky (e.g., marketing) and only applicable to a single disk on a busmastering PCIe controller on a server (doing nothing else on that bus). In the context of a storage system, where you have multiple drives arbitrating & exchanging information in a controlled manner on a FC bus, those transfer times are immaterial. The transfer rates of the bus will only be matched by multiple drives, which will hit maybe 30 MB/s each with a good IO profile. Whether it is 10 drives or 20, it's all the same to the bus. There is a cost to having MORE drives on the bus, but that is a small effect.

Lastly, the proof is always in the pudding. If SATA drives 'slowed down' the bus then you'd wonder why we hit some of our higherst bandwidth numbers WITH SATA DRIVES. And, we do.

So, please do the reading yourself. Place drives on buses based on general aggregation of workload - if you have very different workload profiles then you might separate groups of disk by bus. Certainly do so if you have disks rated at different interface speeds (e.g., 2 Gb/s vs. 4 Gb/s). But in the majority of cases, mixing SATA and FC in an of itself has no effect in the real world.