Oops, what does (F) in /proc/mdstat means ??

Hi All, a recently typed less /proc/mdstat showed the following: less /proc/mdstat Personalities : [linear] [raid0] [raid1] [raid5] read_ahead 1024 sectors md0 : active raid5 sde1[3] sdd1[2] sdc1[1](F) sdb1[0] 5810304 blocks level 5, 64k chunk, algorithm 2 [4/3] [U_UU]

How to test raid5 performance best ?

Hello All, some day ago i joined the Software-Raid-Club :) I'm now running a SCSI-Raid5 with 3 2 GB partitions. I choosed a chunk-size of 32 kb. Referring to the FAQ i'm told to experiment to get best performance chunk-size, but i definitly have no good clue how to test performace :-/ so please