how does sectsz is determined ?

[Raz]

How does sectsz determined to be 512 or 4096 ?
I have an xfs file system over software raid , and sometimes I have
1/2K sector size
and sometimes 4K . Problem is that direct io buffers are aligned to
1/2k and not 4K.
kernel is 2.6.18-8.el5. thank you.
raz


meta-data=/dev/md5               isize=256    agcount=33, agsize=10992384 blks
         =                       sectsz=4096  attr=0
data     =                       bsize=4096   blocks=351780864, imaxpct=25
         =                       sunit=768    swidth=768 blks, unwritten=1
naming   =version 2              bsize=4096
log      =internal               bsize=4096   blocks=32768, version=2
         =                       sectsz=4096  sunit=1 blks
realtime =none                   extsz=3145728 blocks=0, rtextents=0