Need SW RAID5 & mkfs.xfs help with chunksize & swidth/su respectively

[Justin Piszcz <jpiszcz@xxxxxxxxxxxxxxx>]

Two questions, a SW RAID and XFS question.

SW RAID Question: What is the optimal chunk size for a RAID5 that will be 
used with XFS?  I have 5x400GB Seagate HDDs (ATA/100) w/ 8MB of Cache, 
each is a master on its own IDE bus.

From the mkfs.xfs man page:

              Otherwise, the size suboption is only needed if the log  section
              of  the filesystem should occupy less space than the size of the
              special file.  The size is specified in bytes or blocks, with  a
              b suffix meaning multiplication by the filesystem block size, as
              described above.  The overriding minimum value for size  is  512
              blocks.   With some combinations of filesystem block size, inode
              size, and directory block size, the minimum log size  is  larger
              than 512 blocks.

              Using  the  version suboption to specify a version 2 log enables
              the sunit suboption, and allows the  logbsize  to  be  increased
              beyond  32K.  Version 2 logs are automatically selected if a log
              stripe unit is specified.  See sunit and su suboptions, below.

              The sunit suboption specifies the alignment to be used  for  log
              writes.   The  suboption  value  has to be specified in 512-byte
              block units.  Use the su suboption to  specify  the  log  stripe
              unit  size  in bytes.  Log writes will be aligned on this bound-
              ary, and rounded up to this boundary.  This gives major improve-
              ments  in  performance  on  some configurations such as software
              raid5 when the sunit is specified as the filesystem block  size.
              The  equivalent  byte value must be a multiple of the filesystem
              block size.  Version 2 logs are automatically  selected  if  the
              log su suboption is specified.

              The  su suboption is an alternative to using sunit.  The su sub-
              option is used to specify the log stripe.  The  suboption  value
              has  to  be  specified  in bytes, (usually using the s or b suf-
              fixes).  This value must be a multiple of the  filesystem  block
              size.   Version  2 logs are automatically selected if the log su
              suboption is specified.


I have a software raid5 with 5 400GB drives and currently a 512kb 
chunksize with default mkfs.xfs parameters.

p34:~# mdadm --create /dev/md3 --verbose --chunk=512 --level=5 
--raid-devices=5 /dev/hda1 /dev/hde1 /dev/hdg1 /dev/hdi1 /dev/hdk1
mdadm: layout defaults to left-symmetric
mdadm: size set to 390708736K
mdadm: array /dev/md3 started.
p34:~#

What parameters for the sunit/sw are optimal for my configuration?  How 
exactly do I calculate them?

For ext2/ext3 (which I don't use) you use the -stride option and then a 
number for how many disks the filesystem is split over.  For XFS, what is 
the method to calculate these numbers?

Could someone please offer some advice? :)