Re: [PATCHES] O_DIRECT for WAL writes

On Thu, 23 Jun 2005, Tom Lane wrote:

> [ on the other point... ]
>
> Curt Sampson <cjs(at)cynic(dot)net> writes:
> > But is it really a problem? I somewhere got the impression that some
> > drives, on power failure, will be able to keep going for long enough to
> > write out the cache and park the heads anyway. If so, the drive is still
> > guaranteeing the write.
>
> If the drives worked that way, we'd not be seeing any problem, but we do
> see problems. Without having a whole lot of data to back it up, I would
> think that keeping the platter spinning is no problem (sheer rotational
> inertia) but seeking to a lot of new tracks to write randomly-positioned
> dirty sectors would require significant energy that just ain't there
> once the power drops. I seem to recall reading that the seek actuators
> eat the largest share of power in a running drive...

I've seen discussion about disks behaving this way. There's no magic:
they're battery backed.

Thanks,

Gavin