[dm-devel] multipath: Determine which device to use for grubdevice.map file

[berthiaume_wayne at emc.com]

Hi David.

	For SLES you should be using
/dev/disk/by-id/36006016094511600449d92ae1937d611-part2 for a typical
root partition.

Regards,
Wayne. 

-----Original Message-----
From: dm-devel-bounces at redhat.com [mailto:dm-devel-bounces at redhat.com]
On Behalf Of David Huffman
Sent: Thursday, February 18, 2010 8:15 PM
To: dm-devel at redhat.com
Subject: [dm-devel] multipath: Determine which device to use for
grubdevice.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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