Re: RAID10 Layouts

[Goswin von Brederlow <goswin-v-b@web.de>]

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.

> 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.

So to be bootable even with a failed drive you should partition the
disk. Create a small raid1 for the system and a large raid10 for the
data.

> What about this Intel firmware 'RAID'?  Would this assist in any
> way?  How does it relate (if it does) to the linux md system?
> Should I set in BIOS to RAID, or leave it at ACPI?

I would stay away from any half baked bios stuff. It will be no better
than linux software raid but will tie you to the specific bios. If
your mainboard fails and the next one has a different bios you can't
boot your disks.

> 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.

MfG
        Goswin