Subject: Re: Interactive responsiveness under heavy I/O load
To: Daniel Carosone <firstname.lastname@example.org>
From: Johnny Billquist <email@example.com>
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 <firstname.lastname@example.org> 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: email@example.com || Reading murder books
pdp is alive! || tryin' to stay hip" - B. Idol