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Go Back   Linux Archive > Redhat > Device-mapper Development

 
 
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Old 02-19-2010, 12:14 AM
David Huffman
 
Default multipath: Determine which device to use for grub device.map file

I need to configure a dm-multipath device (mpatha) as a boot disk in a
script. For RHEL it appears I can put the mpath device in my device.map
file. However, for SLES 11 if I use the mpath device, grub fails with
"Floating point exception". For SLES I have to use the underlying path
device (sd[a-z]).


I know which devices make up my multipath disk. Here is an example that
shows my problem.


mpatha (36006016094511600449d92ae1937d611) dm-0 DGC,DISK
[size=50G][features=1 queue_if_no_path][hwhandler=1 emc][rw]
\_ round-robin 0 [prio=2][active]
\_ 2:0:1:0 sdd 8:48 [active][ready]
\_ 3:0:1:0 sdf 8:80 [active][ready]
\_ round-robin 0 [prio=0][enabled]
\_ 2:0:0:0 sdc 8:32 [active][ready]
\_ 3:0:0:0 sde 8:64 [active][ready]

Out of the 4 paths, only two (sdd and sdf) are active. This output is
not very good to parse through in a script to determine which disks I
can specify in my device.map file. Is there a command or better way to
determine which underlying paths I can use in my device.map file.


For clarification, here is a breakdown of my device.map entries and the
results:


(hd0) /dev/mapper/mpatha - FAIL with "Floating point exception"

(hd0) /dev/sdc - FAIL with "Error 21: Selected disk does not exist"

(hd0) /dev/sdd - Success!!!

I would rather put the mpath device in device.map, but that does not
appear to be an option for SLES. Is there a way to determine the
underlying path device that is "active" and not just enabled?


Right now I can run "dd if=/dev/sdc of=/dev/null bs=512 count=1
2>/dev/null; echo $?"


This will tell me in the device can be written to. Any suggestions would
be helpful.


- David

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