[rsyslog] Development of failsafe disk based queue

[Rainer Gerhards]

On Wed, 2008-10-01 at 06:35 -0700, david at lang.hm wrote:
> On Wed, 1 Oct 2008, david at lang.hm wrote:
> 
> > On Wed, 1 Oct 2008, Rainer Gerhards wrote:
> >
> >> Even then, in the worst case, I think it would be possible that the disk
> >> does only a partial write. I am not sure if that's really the case with
> >> today's disk drives (which I think have capacitors to prevent this
> >> scenario), but with past drives this could happen (I know all too well -
> >> a few years ago that cost me a weekend ;)).
> >
> > current disks do not have capacitors to prevent partial writes or to flush
> > their caches. but options like the linux ext3 data-journaled make it so
> > that you have your data in the journal safely, and the various solid-state
> > options solve that problem.
> 
> actually, I need to correct my answer.
> 
> I know that disks do not have capacitors large enough to write their 
> buffer.

absolutely
> 
> I'm pretty sure that they don't have capacitors large enough to write an 
> entire track.
> but they may have capacitors large enough to finish the sector they are on 
> before stopping (considering the number of sectors in a track nowdays this 
> is a _very_ sort time)

*That* I had expected. Especially if you think about the low-level sync marks
which otherwise could be affected. I think loss of disk could otherwise be
the extreme result of a power failure (or at least loss of disk track). But
you never know what a failing write arm does...

So I would think (but have no evidence) that current disk drives have
capacitors large enough to finish the write AND shut down the write
mechanism in an orderly manner.

As you say, there are "not many electrons" required to ensure this.

Rainer