Ramblings from Richard's Ranch

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January 23, 2009

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I'm continuing my previous blog on this new site.

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IOPS and latency are not related - HDD performance explored

Today, we routinely hear people carrying on about IOPS-this and IOPS-that. Mostly this seems to come from marketing people: 1.5 million IOPS-this, billion IOPS-that. Right off the bat, a billion IOPS is not hard to do, the metric lends itself rather well to parallelization... This post is the first in a series looking at the use and misuse of IOPS for storage system performance analysis or specification. Let's do some simple math. We all want low latency -- the holy grail of performance. In the bad old days, many computer systems were bandwidth constrained in the I/O data path, so it was very easy to measure the effect of bandwidth constraints on latency. For example, fast/wide parallel SCSI and UltraSCSI was the rage when the dot-com bubble was bubbling, capped out at 20 MB/sec. Suppose we had to move 100 MB of data, then the latency is easily calculated: Bad Old Latency = amount of stuff / bandwidth = 100 MB / 20MB/sec = 5 sec Well, there it is, 5 seconds later. For ...

Tour goes to Palomar

This year the AMGEN Tour of California is (finally) coming to San Diego County to climb Palomar Mountain . Palomar Mountain is not too far from the Ranch, and the Palomar Volunteer Fire Department has put out the call for volunteers to help the department with whatever might happen as thousands of race fans converge on the mountain. The Ramona Outback Amateur Radio Society and Ramona-CERT is providing volunteers to help with communications for the department. I've spent quite a few hours recently with the fine folks on Palomar Mountain helping to coordinate communications for the event. Today, we held a pre-race communications exercise as part of the 2009 San Diego County CERT Spring Exercise. Dozens of hams and CERT folks surveyed and characterized communications capabilities on the mountain. What is surprising to many people who don't regularly travel to the back country is that cell phone service is likely to be non-existent. And if you have a need for emergency respons...

ZFS data protection comparison

ZFS now offers triple-parity raidz3. Conceptually, raidz3 is an N+3 parity protection scheme. Today, there are few, if any, other implementations of triple parity protection, so when we say "raidz is similar to RAID-5" and "raidz2 is similar to RAID-6" there is no similar allusion for raidz3. I prefer to say "raidz3 is like raidz2 with one additional level of parity protection. But how much better is raidz3 than raidz2? To help answer that question, I used the simple Mean Time to Data Loss (MTTDL) model to calculate the data retention capabilities of the possible configurations of 12 disks under ZFS. To be fair, the same model applies to other RAID implementations, but I'll use the ZFS terminology here. In this MTTDL model, the configuration includes N total disks. If the data protection scheme is raidz3, then the minimum N = 1 data disk + 3 parity disks = 4. You can add more data disks to increase the overall available space, so if N=6 then you have 3 da...

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