Re: Solaris ZFS on Linux [Was: Re: the " 'official' point of view"expressedby kernelnewbies.org 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
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.
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
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.
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...
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/