Re: Solaris ZFS on Linux [Was: Re: the " 'official' point of view"expressed by kernelnewbies.org regarding reiser4 inclusion]

[Ian Stirling <ian.stirling@mauve.plus.com>]

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.

You probably don't actually want to flush the cache - but to write
to a journal.
16M of cache - split into 32000 writes to single sectors spread over
the disk could well take several minutes to write. Slapping it onto
a journal would take well under .2 seconds.
That's a non-trivial amount of storage though - 3J or so, 40mF@12V -
a moderately large/expensive capacitor.

And if you've got to spin the drive up, you've just added another
order of magnitude.

You can see why a flash backup of the write cache may be nicer.
You can do it if the disk isn't spinning.
It uses moderately less energy - and at a much lower rate, which
means the power supply can be _much_ cheaper. I'd guess it's the
difference between under $2 and $10.
And if you can use it as a lazy write cache for laptops - things
just got better battery life wise too.
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