Re: multipath: Determine which device to use for grubdevice.map file

[David Huffman <dm-devel@storix.com>]

Thanks for the response Wayne, but this does me no good. You cannot use 
the /dev/disk/by-id/* entries for device.map. You have to use the block 
device and not a link or dm-* device name.

Essentially, I need to know which underlying paths can be written to 
directly.

-David

berthiaume_wayne@emc.com wrote:
> 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@redhat.com [mailto:dm-devel-bounces@redhat.com]
> On Behalf Of David Huffman
> Sent: Thursday, February 18, 2010 8:15 PM
> To: dm-devel@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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