Subject: Re: Interactive responsiveness under heavy I/O load
To: Daniel Carosone <>
From: Johnny Billquist <>
List: tech-kern
Date: 01/26/2004 23:43:18
On Tue, 27 Jan 2004, Daniel Carosone wrote:

> On Mon, Jan 26, 2004 at 02:12:24PM -0600, Johan A. van Zanten wrote:
> >
> > John Goerzen <> wrote:
> >
> > > Have I stumbled across the reason softdep is not enabled by default, or
> > > is there some other logic behind this?
> >
> > My understanding of how soft dependencies works is that file system
> > metadata is cached in memory for a short period of time (like 20
> > seconds?), which means it's at risk (of being lost) if the machine
> > were to loose power.
> Two points:
>  - The question of "on by default" is shady; softdeps aren't enabled
>    unless you specify the mount option, but I understand sysinst now
>    creates fstab's with that option by default.  So most users would
>    have them enabled on new systems at least. I may be wrong about
>    sysinst, its not something I really ever use.
>  - You are right about the metadata being delayed, but it's still
>    written in-order and before the relevant file data. The essential
>    point of softdep is to allow the ordering up updates to the on-disk
>    data structures to be preserved, so the ffs and fsck semantics that
>    depend on these still work, but without requiring synchronous
>    writes that make everything stop and wait. If you sync or fsync at
>    the right time, the previous semantics about data on disk or not
>    still hold.
>    Speaking very broadly, if your system crashes, the resulting
>    filesystem state with softdep is similar to if the machine had
>    crashed a little earlier without (unless you sync).
>    Softdep is such a huge performance win, especially for tasks like
>    extracting a pkgsrc tree, because lots of metadata writes update
>    the same disk block repetitively (think about adding files one at a
>    time to the same directory). With softdep, each of these updates is
>    done in memory (without the sync disk wait) and the resulting final
>    directory blocks written to disk (again, speaking very broadly).

softdep:s really sounds nice, but last I tried it (a few months ago) it
still crashed my VAX very predictably, so I'd guess it still isn't good
enough to turn on for people in general.


Johnny Billquist                  || "I'm on a bus
                                  ||  on a psychedelic trip
email:           ||  Reading murder books
pdp is alive!                     ||  tryin' to stay hip" - B. Idol