Re: Improvements to fsck performance in -current ...?

> Date: Wed, 01 Oct 2003 06:39:47 +0000
> From: Jens Rehsack <rehsack_at_liwing.de>
> 
> Kevin Oberman wrote:
> 
> [...]
> 
> > Current has two major changes re speeding up fsck.
> > 
> > The most significant is the background operation of fsck on file
> > system with soft updates enabled. Because of the way softupdates
> > works, you are assured of metadata consistency on reboot, so the file
> > systems can be mounted and used immediately with fsck started up in
> > the background about a minute after the system comes up.
> 
> Be careful what you promise :-)
> Most new disks have an own disk cache and some of them have a
> write cache enabled. In case of a hardware failure (or power
> failure) this data may get lost and the disk's metadata isn't
> consistent. It's only when no write cache below the system
> is active.

Yes. This is fairly well known with many, many messages in the
archives. If you want serious stability, you need to turn off the disk
write cache. I have it off on my office system here and on on my
laptop.

But thanks for bringing this up as it is important. And, yes, it has
burned me, although it required a confluence of things all going wrong
at exactly the right timing to catch a bunch of metadata in cache.
(This could only have happened on a CURRENT system back in the 5.0
time frame.) It could only happen when the file system had been very
active with an installworld. But it did happen.

The trade-off is a big performance hit. With disk cache on, I can copy
my entire FreeBSD partition to another disk in about 15 minutes. With
disk cache off, it took a few HOURS. This was a worst case example
with dd on my laptop (slow disks).
-- 
R. Kevin Oberman, Network Engineer
Energy Sciences Network (ESnet)
Ernest O. Lawrence Berkeley National Laboratory (Berkeley Lab)
E-mail: oberman_at_es.net			Phone: +1 510 486-8634