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