Re: RAID10 Layouts

["Keld Jørn Simonsen" <keld@dkuug.dk>]

On Fri, Aug 21, 2009 at 06:43:28PM +0200, Goswin von Brederlow wrote:
> Info@quantum-sci.net writes:
> 
> > 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.

> > 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

> > 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