Re: Solaris ZFS on Linux [Was: Re: the " 'official' point of view"expressedby regarding reiser4 inclusion]

From: David Masover
Date: Tue Aug 01 2006 - 00:30:22 EST

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.

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.

Very useful for a laptop -- I used to do this in Linux by using Reiser4, setting the disk to spin down, and letting lazy writes do their thing, but I didn't have enough RAM, and there's always the possibility of losing data. But leaving the disk off is nice, because in the event of sudden motion, it's safer that way. Besides, most hardware gets designed for That Other OS, which doesn't support any kind of Laptop Mode, so it's nice to be able to enforce this at a hardware level, in a safe way.

I've heard to many fancy-sounding drive technologies that never hit the market, I'll wait until thye are actually available before I start counting on them for anything (let alone design/run a filesystem that requires them :-)

Or even remember their names.

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...

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