Messages in this thread | | | Date | Sat, 15 Mar 2008 21:18:16 +0100 | From | Pavel Machek <> | Subject | Re: [ANNOUNCE] Ramback: faster than a speeding bullet |
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Hi!
> > if you have a reliable UPS and are willing to rely on it to save your data > > take the identical hardware to what you are planning to use, but instead > > of using your driver just create a ramdisk and load it on boot and save > > the contents on shutdown. > > Aha! You are getting close. Really, that is all ramback does. It > just handles some very difficult related issues efficiently, in such a > way as to minimize any denial of service from complete loss of UPS > power. This is all just about using power management in a new way that > gets higher performance. But your battery power has to be reliable. > Just make it so. It is not difficult these days, or even particularly > expensive. > > I calculated somewhere along the line that it would take something like > 17 minutes to populate the big Violin ramdisk initially, and 17 minutes > to save it during a loss of line power event, during which UPS power > must be not run out before ramback achieves disk sync or you will get > file corruption. (This rule was mentioned in my original post.) > > All well and could, you can in fact do that with a pretty simple script. > But in the initial 17 minutes your application may not read or write > the ramdisk data and in the closing 17 minutes it may not write. That > knocks your system down to 4 nines, given one planned shutdown per year. > Not good, not good at all.
Hmm, what happens if applications keep dirtying so much data you miss your 17minute deadline?
Anyway... ext2 + lots of memory + tweaked settings of kflushd (only write data older than 10 years) + just not using sync/fsync except during shutdown + find / | xargs cat ...is ramback, right? Should have same performance, and you can still read/write during that 17+17 minutes.
Ok, find | xargs might be slower... but we probably want to fix that anyway....
It has big advantage: if you only tell kflushd to hold up writes for an hour, you loose a little in performance and gain a lot in reliability...
(If ext2+tweaks is slower than ramback, we have a bug to fix, I'm afraid). Pavel -- (english) http://www.livejournal.com/~pavelmachek (cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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