umount xfs filesystem

[Daniele Nuzzo - Readytec SPA]

hi,
I think I need your advice-opinion ...
I set up a system like CentOS 5.3, lvm, drbd 8.2, various systems raid 
(Adaptec aacraid or LSI MegaRAID with or without write back cache) after 
some time of work (a few hours or days) do the umount is slow... even a 
few minutes ... why?
I have already disabled barrier and write cache on drbd, but not on the 
controller or the sas  drives... ideas?
thanks a lot.

p.s.
drbd partitions formatted with the default options.
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