Date:	Fri, 18 Jun 1999 00:20:36 -0700 (PDT)
From:	Linus Torvalds <torvalds@transmeta.com>
To:	"Stephen C. Tweedie" <sct@redhat.com>
Subject: Re: [patch] `cp /dev/zero /tmp' (patch against 2.2.9)



Anybody who is interested in FS performance should take a look at the
latest pre-patch of 2.3.7 (only pre-6 and possibly later: do NOT get any
earlier versions. pre-5 still causes file corruption, pre-6 looks good so
far).

Careful, though: I fixed the problem that caused some corruption less than
an hour ago, and while my tests indicate it all works fine, this is a very
fundamental change. The difference to earlier kernels is:

 - ext2 (and some other block device filesystems that have been taught
   about it) uses write-through from the page cache instead of having a
   separate buffer cache and the page cache to maintain dirty state. This
   means much less memory pressure in certain situations, and it also
   means that we can avoid unnecessary copies.

 - the page cache has been threaded, so on SMP you can actually get
   noticeable speedups from processes that do concurrent file accesses.

 - lower-latency read paths, especially the cached case.

Both of these are big, and fundamental changes. So don't mistake me when I
say it is experimental: Ingo, David and I have been spending the last
weeks (especialy Ingo, who deserves a _lot_ of credit for this all: I
designed much of it, but Ingo made it a reality. Thanks Ingo) on making it
do the right thing and be stable, but if you worry about not having
backups you might not want to play with it even so. It took us this long
just to make it work reliably enough that we can't find any obvious
problems..

The interesting areas are things like
 - writes to shared mappings now go blindingly fast. We're talking mondo
   cleanups here. We used to do really badly on this, now we do really
   well.

 - does bdflush still do the right thing? There may be a _lot_ of tweaking
   to do to get everything working at full capacity.

 - can people confirm that it is stable for everybody?

 - if anybody has 8-way machines etc, scalability is interesting. It
   should scale to 8-way no problem. We used to scale to 1-way, barely.
   Numbers?

 - fsync(). It doesn't work right now, but it should be easy to make it
   work well on big files etc - something we've never been able to do
   before (we used to lack the indexing from file to dirty blocks: now we
   have access to that quite automatically thanks to having the
   inode->page index in place, and the dirty blocks are right there)

and I'd really appreciate comments from people, as long as people are
aware that it _looks_ stable but we don't guarantee anything at this
point. 

		Linus


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