On Tue, Nov 23, 1999 at 02:04:50AM +0000, Florian G. Pflug wrote:
> > I don't really mean to be a hardass here, but you need to know about
> > how the qmail queue works. You have the qmail source right? Included
> > with that source is an "INTERNALS" document which describes how the
> > queue works. With qmail's insistance on fsync'ing, you can see how
> > a writeback cache on the HW RAID controller can help.
> >
> > Or perhaps you don't know? HW RAID controllers can come with non-volitile
> > RAM caches. When part of this cache is in "writeback" mode, scsi write
> > commands are put in the cache, and the controller tells the OS that the
> > command has been completed. Then the writes are committed to hard drive
> > (which have their own caches). Thus, multiple small-block writes followed
> > by fsync's should finish much quicker on a HW RAID with writeback cache.
> >
> > If you're relying on OS RAM to do the same thing for a filesystem, then
> > the fsync will put an end to that.
>
> All this would make hw-cach *forbidden* for qmail queue dir, since then it
> is *not* guaranteed, that what is synced is writted on disk and will
> survive a power loss....
It depends on your raid card. Some cards/systems battery back-up their cache
so that it is not lost in the event of a power failure.
> Anyway, what is noone mentioning raid 5? I just played with it under linux
> (software raid) until now - but it seems quite fast.
What may seem fast to you could be nothing compared to the numbers people are
looking for in large environments.
--Adam
>
> Greetings, Florian Pflug
>
