external log config [was: Million files or so on RAID 5 Partition]

[Stuart Levy <slevy@ncsa.uiuc.edu>]

Simon Matter suggested, re improving setup of a RAID-5:
  [...]
> 2) The really important thing is how you set up the raid5 array. Did you
> make an external log? That makes the BIG difference. I suggest this
> layout:
> 
> Every disk has:
> Part1: 200M	Part2: 8.8G
> create raid1 on disk 1-2 / part 1
> create swap on disk 3-8 / part1
> create raid5 on disk 1-8 / part 2
> 
> create XFS FS on raid5 with external log on raid1

Just wondering about the importance of a redundant
external log.  I have a RAID-5 setup for the main filesystem
data, but keep the log on a nonredundant partition of
another drive, on the theory that if the log were lost,
I shouldn't lose more than the last few seconds' activity.

And it appears that xfs_repair -l /dev/newpartition -L /dev/maindevice
can be used to initialize a new external log -- I won't need
to re-create the entire filesystem or anything.

(I tested the above create-new-log scenario, though only on a
cleanly-unmounted filesystem.)

Also, for performance, I imagine that it'd help a lot to have
an external log on a *different spindle* than the
main filesystem, to avoid lots of extra seeks.
Since N disks fit in my system, I used N-1 for the RAID-5
main xfs filesystem and the other 1 for the external log
(and system disk).  But if the ext log needed to be *both*
redundant and to share no spindles with the main fs,
I'd be limited to N-2 disks for XFS data, a big loss.

Does this sound right?

   Stuart Levy, slevy@ncsa.uiuc.edu