[Beowulf] OT: Software RAID & Multipath

[Chris Samuel]

On Tuesday 09 January 2007 10:01, Chris Samuel wrote:

> 1) Use software RAID-0 with MD.  My concern then is that I don't know
> whether the RAID-0 will kick in *before* the multipath and think it can
> stripe over 4 drives (which would be bad).

I gave it a go myself and the easiest solution is the above, just used mdadm 
to create a pair of multipath RAID devices (md0 and md1) for each path to the 
visible LUNs, then mdadm'd a RAID-0 device (md2) over md0 and md1.

Then pvcreate /dev/md2, vgcreate RAID /dev/md2 and away you go with sharpened 
mallets..

I cribbed the MD multipath stuff from:

  http://oss.gonicus.de/openpower/index.php/Dm-multipath-vscsi

which I found after posting the original email.

I did try using multipath-tools, but LVM was starting up before multipath (as 
I was using LVM for the Ubuntu install proper) and it was grabbing the raw 
devices, noticing the duplicate pv labels and only picking one SCSI device of 
each pair to use.

I could have hacked around it, but I wanted something that just worked and 
wouldn't catch us out later after a dist-upgrade and reboot..

cheers,
Chris
-- 
 Christopher Samuel - (03)9925 4751 - VPAC Deputy Systems Manager
 Victorian Partnership for Advanced Computing http://www.vpac.org/
 Bldg 91, 110 Victoria Street, Carlton South, VIC 3053, Australia

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