ufs+softupdates / consistency

[Arne Wörner]

--- David Schultz <das at FreeBSD.ORG> wrote:
> ext2fs mounted async does not provide consistency; in fact the
> state of the disk can be almost arbitrarily inconsistent at any
> given time.  Soft updates is supposed to provide performance
> comparable to async writes without the inconsistency problem. 
> I'm not sure what it is about your setup that causes such a
> disparity. (Many factors such as the FS block size and ATA write
> caching can make a big difference.)
>
Somebody in list freebsd-performance@ opened a thread "FreeBSD 5.3
I/O Performance / Linux 2.6.10 | Continued Discussion", where he
and one other state, that FreeBSD R4.11 beforms much better (twice
faster) than FreeBSD R5.3.

Could the disparity I saw be caused by the SMPng project in R5.3?

I have to admit, that I never tested ext2fs but ext3fs with
unknown fs parameters (I just used mkfs_ext3fs (or so?)).

> By the way, ext3fs uses journalling, which provides metadata
> consistency in a very different way from soft updates.  You
> might also want to experiment with that to see if it works
better
> for your workload.
>
Hmm... I do not understand this hint.

Does FreeBSD offer a journaling file system?

I liked the idea of soft updates quite much, because it saves disc
write accesses (as far as I understood it), so that I would like
to continue to use ufs+softupd...

> > Are we already trying to issue write order requests for the
> > disc blocks (whose write order is arbitrary) sorted by sector
> > number (in order to move the disc heads as less as possible)?
> > The disc write cache could do that, but I disabled it in order
> > to decrease the probability of inconsistency.
> 
> Hopefully you disabled it on both FreeBSD and Linux, so you're
> comparing apples to apples...
> 
During the tests I enabled write cache in both settings, because I
did not know how to turn it off in KNOPPIX...

Isn't it possible to simulate the hard disc write cache in kernel?

I could try to write some code, but I am quite unfamiliar with
kernel program writing.

Maybe this is not a fs@ subject but more a performance@ subject?

-Arne



		
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