Very low disk performance on 5.x

[Poul-Henning Kamp]

In message <003901c54f36$0c64ad40$b3db87d4 at multiplay.co.uk>, "Steven Hartland" 
writes:

>Ok from what your saying it sounds like RAID on FreeBSD is useless
>apart to create large disks. Now to the damaging facts the results
>from my two days worth of testing:

Now, cool down a moment and lets talk about what you _really_ have
measured.

You have measured the end to end performance: you include /dev/zero,
dd(1), filesystem, disk device driver, hardware and disks.

As such that is a fair end-user benchmark, but unfortunately it
doesn't really tell us anything useful for the purpose of this
discussion.

If you are going to do a high-performance setup, you should not
just take the "out-of-box" settings, you should optimize for
the configurations particular features.

Testing end-to-end means that we have very little to go from to
find out where things went wrong in any one instance.

But anyway: here are some questions which I wonder about ?

Does the linux and FreeBSD filesystem offer the same semantics
with respect to integrity ?  Ie: if one is asyncronous mode
the comparison is not a fair comparison.   Just because you
chose the default in each case doesn't mean you got the same
thing.

Does any of the drivers change the settings/modes of the controllers
use of cache ?  (this may be hard to determine without looking 
at driver sources.

In particular, are you sure that the RAID-5 logical device was
in the same exact state and location for each run ?

Did you remember to disable all the debugging in FreeBSD 6-Current ?
(see top of src/UPDATING)


-- 
Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
phk at FreeBSD.ORG         | TCP/IP since RFC 956
FreeBSD committer       | BSD since 4.3-tahoe    
Never attribute to malice what can adequately be explained by incompetence.