cost of tdb transactions on ext4 with barriers

[tridge at samba.org]

Hi Rusty,

for completeness, here is are the results including the btrfs and xfs
numbers:

   ext3 barrier=0   1.9s
   ext3 barrier=1   13.4s

   ext4 barrier=0   2.5s
   ext4 barrier=1   36.0s

   btrfs nobarrier  29.1s  
   btrfs barrier=1  117s

   xfs   	    8.9s  

(the barrier options make no difference on xfs as far as I can see).

If ext4 and btrfs are the future of Linux filesystems then tdb/Samba
are in real trouble!

Cheers, Tridge