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 [email protected]
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
[Index of Archives]
[Kernel Newbies]
[Netfilter]
[Bugtraq]
[Photo]
[Stuff]
[Gimp]
[Yosemite News]
[MIPS Linux]
[ARM Linux]
[Linux Security]
[Linux RAID]
[Video 4 Linux]
[Linux for the blind]
[Linux Resources]
