planet-ltc.org

I picked up a 1 TB Western Digital My Book World from Fry's today - that's a 1 Terabyte Gigabit Network Attached Storage box - for $220. I've seen a few consumer technical appliances that run Linux under the covers, and haven't been terribly impressed with many of them. So far however, this box is slick. While WD doesn't support Linux officially (bad WD!) it is pretty trivial to get the box into a techie-friendly state.

With only an hour or so of tinkering, I was able to enable ssh, disable the java server, add my own users, and mount my partitions via sshfs. The info on doing this is already covered by "Paul" at his How to setup My Book World Edition II page so I won't duplicate that here. Should also probably link to Martin Hinner's My Book Howto directly as he appears to have done the hard work. I'll spend the next few days figuring out how to best make use of this device; most likely looking into something like rdiff-backup.

Something they don't mention are the specs of this little beauty, for those of you who might care, read on...

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Today at the Red Hat Summit in Boston, Red Hat announced the official release of Red Hat Enterprise MRG V1 (Messaging Realtime Grid) [1]. A couple snippets of note:

"The Realtime component of Red Hat Enterprise MRG comprises numerous kernel enhancements that provide deterministic performance for time-critical and latency-sensitive applications."

"IBM has worked together closely with Red Hat on the development of the real time Linux kernel and has optimized both WebSphere Real Time and BladeCenter servers on Red Hat Enterprise MRG. We are delighted that IBM and Raytheon have been recognized by Red Hat for this innovation which has led to the largest deployment of real time technology in the next generation of US Navy Destroyers." --Jeff Smith, vice president, Open Source and Linux Middleware at IBM

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As enterprise distributions continue to roll out their respective real-time offerings, the benefits (and drawbacks) of real-time are becoming available to the masses. The expectations for these distros are often a mixture of general purpose computing performance and real-time determinism and low-latency. Unfortunately, meeting one of these often comes at the expense of the other. Deterministic behavior requires more overhead in the OS, including some lock serialization and additional context switching, which naturally degrades throughput. Two real-time Linux developers, Steven Rostedt of Red Hat and Gregory Haskins of Novell have been actively working on two distinct approaches to minimizing the effects of determinism on throughput.

Steven's rwlock-multi patches [1] (2.6.24.X-rt8) attempt to reduce the bottleneck of the conversion of the rwlock to an rtmutex in the PREEMPT_RT [2] tree. In order to provide Priority Inheritance [3] support, the rwlock was converted to an rtmutex, limiting it to one owner. This bottleneck hit some subsystems harder than others, networking was particularly affected. The rwlock-multi patch relieves the serialization by allowing a fixed number (configurable) of readers to hold the lock at any one time, bounding the wait time to a higher priority writer. As would be expected, if a high priority writer is blocked on an rwlock, it will boost the priority of all the readers, and prevent any new lower priority readers from getting the lock (as soon as the existing readers release the lock, the writer gets it). This approach certainly increases the latency a high priority writer may see when acquiring a contended lock, but the increase is bounded, and therefor still deterministic. The benefit is seen by all subsystems that leveraged the rwlock for additional throughput, bringing the rt kernel closer in performance to the mainline kernel, while maintaining the necessary deterministic behavior.

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