On Fri, Aug 21, 2009 at 06:43:28PM +0200, Goswin von Brederlow wrote:

Info@quantum-sci.net writes:

quoted

Hello list,

Researching RAID10, trying to learn the most advanced system for a 2
SATA drive system.  Have two WD 2TB drives for a media computer, and
the most important requirement is data redundancy.  I realize that
RAID is no substitute for backups, but this is a backup for the
backups and the purpose here is data safety.  The secondary goal is
speed enhancement.  It appears that RAID10 can give both.

First question is on layout of RAID10.  In studying the man pages it
seems that Far mode gives 95% of the speed of RAID0, but with
increased seek for writes.  And that Offset retains much of this
benefit while increasing efficiency of writes.  What should be the
preference, Far or Offset?  Are they equally as robust?

All raid10 layouts offer the same robustness. Which layout is best for
you really depends on your use case. Probably the biggest factor will
be the average file size. My experience is that with large files the
far copies do not cost noticeable write speed while being twice as
fast reading as raid1.

The file system elevator makes up for the Far write head movement.

quoted

How safe is the data in Far or Offset mode?  If a drive fails, will
a complete, usable, bootable system exist on the other drive?
(These two are the only drives in the system, which is Debian
Testing, Debian kernel 2.6.30-5) Need I make any special Grub
settings?

I don't think lilo or grub1 can boot from raid10 at all with offset or
far copies. With near copies you are identical to a simple raid1 so
that would boot.

there is a howto on setting up a system, that can continue runnig, if one 
disk fails at
http://linux-raid.osdl.org/index.php/Preventing_against_a_failing_disk

quoted

How does this look:
# mdadm --create /dev/md0 --level=raid10 --layout=o2 --metadata=1.2 --chunk=64 --raid-disks=2 missing /dev/sdb1

On partitions it is save to use 1.1 format. Saves you 4k. Jupey.

You should play with the chunksize though and try with and without
bitmap and different bitmap sizes. Bitmap costs some write performance
but it greatly speeds up resyncs after a crash or temporary drive
failure.

I would recommend a bigger chunk size. at least 256 kiB.

Best regards
keld