Very low disk performance on 5.x

[Poul-Henning Kamp]

>Interesting stuff so:
>1. How to we test if this is happening?

Calculate by hand what the offset of the striped/raid part of the disk
is (ie: take slice+partition stats into account).

>2. How do we prevent it from happening?

Make sure that the first sector of a partition/slice is always the first
sector in a stripe on your raid/stripe/whatever.

>3. Why would this be effecting reads and not writes as surely the same
>blocking is being done for both?

Write on RAID5 uses a cache which lies to you about when things are
safely stored on the disk.

Good RAID5 has battery backup for that cache.

The MBR slice format is stupid because it more often than not gets
this exactly wrong.  Typically there are 63 "sectors per track" and
that ruins any alignment in 99% of the cases.

Sysinstall, fdisk and bsdlabel should know about all this and try
to help the user get it right.  Fixing them to do so may be more
trouble than writing a better too bottom up.

-- 
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.