Ramblings from Richard's Ranch

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As we're getting ready for summer at the ranch...

June 01, 2016

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Lion's tails reach for the sky!

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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

On ZFS Copies

JRS Systems blogs about ZFS copies at http://jrs-s.net/2016/05/09/testing-copies-equals-n-resiliency/ I tried to reply there, but the website wouldn't accept my reply, complaining about cookies and contact the site administrator. So I'll reply here. Internet Hurrah! For a single device pool, copies=2 places the redundant copies approximately 1/3 (copies=2) and 2/3 (copies=3) into the LBA range of the single device. Assuming devices allocate with some diversity by LBA, this allows recovery from a range of LBA failures. For HDDs, think head-contacts-media type of failures. For a random failure case, you get random failures. By contrast, if the pool has two top-level vdevs, such as a simple 2-drive stripe, then the copies are placed on separate drives, if possible. In this case, copies=2|3 provides protection more similar to mirroring, where the copies are on diverse devices. It is not identical to mirroring, because the pool itself depends on all top-level vdevs func

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As we're getting ready for summer at the ranch...

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