Problem identified: WAPL/RAIDframe performance problems

[Edgar Fuß <ef%math.uni-bonn.de@localhost>]

> I seem to be facing two problems:
> 
> 1. A certain svn update command is ridicously slow on my to-be file server.
> 2. During the svn update, the machine partially locks up and fails to respond 
>    to NFS requests.
Thanks to very kind help by hannken@, I now at least know what the problem is.

Short form: WAPBL is currently completely unusable on RAIDframe (I always 
suspected something like that), at least on non-Level 0 sets.

The problem turned out to be wapbl_flush() writing non-fsbsize chunks on non-
fsbsize boundaries. So RAIDframe is nearly sure to RMW.
That makes the log being written to disc at about 1MB/s with the write lock 
on the log being held. So everything else on that fs tstiles on the log's 
read lock.

Anyone in a position to improve that? I could simply turn off logging, but then 
any non-clean shutdown is sure to take ages.