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On Mon, 31 Jul 2006, David Masover wrote: > David Lang wrote: >> On Mon, 31 Jul 2006, David Masover wrote: >> >>> Oh, I'm curious -- do hard drives ever carry enough battery/capacitance to >>> cover their caches? It doesn't seem like it would be that hard/expensive, >>> and if it is done that way, then I think it's valid to leave them on. You >>> could just say that other filesystems aren't taking as much advantage of >>> newer drive features as Reiser :P >> >> there are no drives that have the ability to flush their cache after they >> loose power. > > Aha, so back to the usual argument: UPS! It takes a fraction of a second to > flush that cache. which does absolutly no good if someone trips over the power cord, the fuse blows in the power supply, someone yanks the drive out of the hot-swap bay, etc. >> now, that being said, /. had a story within the last couple of days about >> hard drive manufacturers adding flash to their hard drives. they may be >> aiming to add some non-volitile cache capability to their drives, although >> I didn't think that flash writes were that fast (needed if you dump the >> cache to flash when you loose power), or that easy on power (given that you >> would first loose power), and flash has limited write cycles (needed if you >> always use the cache). > > But, the point of flash was not to replace the RAM cache, but to be another > level. That is, you have your Flash which may be as fast as the disk, maybe > faster, maybe less, and you have maybe a gig worth of it. Even the bloatiest > of OSes aren't really all that big -- my OS X came installed, with all kinds > of apps I'll never use, in less than 10 gigs. > > And I think this story was awhile ago (a dupe? Not surprising), and the > point of the Flash is that as long as your read/write cache doesn't run out, > and you're still in that 1 gig of Flash, you're a bit safer than the RAM > cache, and you can also leave the disk off, as in, spinned down. Parked. as I understand it flash reads are fast (ram speeds), but writes are pretty slow (comparable or worse to spinning media) writing to a ram cache, but having a flash drive behind it doesn't gain you any protection. and I don't think you need it for reads >> external battery backed cache is readily available, either on high-end raid >> controllers or as seperate ram drives (and in raid array boxes), but >> nothing on individual drives. > > Ah. Curses. > > UPS, then. If you have enough time, you could even do a Software Suspend > first -- that way, when power comes back on, you boot back up, and if it's > done quickly enough, connections won't even be dropped... remember, it can take 90W of power to run your CPU, 100+ to run your video card, plus everything else. even a few seconds of power for this is a very significant amount of energy storage. however, I did get a pointer recently at a company makeing super-high capcity caps, up to 2600F (F, not uF!) in a 138mmx tall 57mm dia cyliner, however it only handles 2.7v (they have modules that handle higher voltages available) http://www.maxwell.com/ultracapacitors/index.html however I don't see these as being standard equipment in systems or on drives anytime soon David Lang - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/ | ||||||||||||
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