Subject: Re: disk caching (was: Re: Really bad performance when doing newfs)
To: Chuck Swiger <>
From: Dieter <>
List: netbsd-users
Date: 02/04/2006 15:32:18
> >> This makes predictable and reliable crash recovery of on-disk data structures
> >> (filesystems, databases etc.) impossible.
> > 
> > I was under the impression that filesystems had the ability to force
> > the order of writes when they needed to?  Am I misremembering?
> The filesystem can attempt to enforce write order.

The quote from FreeBSD makes it sound like the filesystem cannot
enforce write order, unless one gets rid of block devices.
This just doesn't sound right.

> > Is this a problem in NetBSD?
> It's an issue which affects all operating systems.

I'm assuming that by "issue" you are referring to the drive lying about
a write being complete?

By "problem", I meant, can the NetBSD filesystem code enforce write order?
(assuming that the disk is not lying about writes being complete)

> Unfortunately, most IDE drives will reorder writes internally and will lie about
> whether they have actually completed transactions.  This is true for many drives
> even if you tell the drive not to enable the write cache.

Are you saying some drives totally ignore the "turn the write cache off"
command?  This is easy to test for.  I see a 10x difference in write
performance between the write cache being on or off.  Or is there some
other problem?