Multipath on boot and root partition with Redhat5.0

Multipath on boot and root partition with Redhat5.0

[Sandrine Twardy]

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Hello,

I have a SAN Eva 5000 HP And I have installed a redhat 5.0 on my server 
with "linux mpath", so far all is fine.
I have an os device multipathed as follow :

 > multipath -ll
mpath0 () dm-0 COMPAQ,HSV110 (C)COMPAQ
[size=7.0G][features=0][hwhandler=0]
\_ round-robin 0 [prio=0][active]
\_ 3:0:0:1 sdb 8:16  [active][ready]
\_ 4:0:0:1 sdc 8:32  [active][ready]

But when I presents a new lun to my host and scans again the scsi lun, I 
am losing the multipath on the disk (/ and /boot)
(possibly because the name of device is changed from sdb/sdc to sdc/sde ? )

Here's an extract of my multipath.conf:

multipath {
              wwid                    3600508b40000c1c50001c000000b0000
              path_grouping_policy    group_by_serial
              rr_weight               uniform
              prio_callout            /bin/true
              alias                   os_lun
      }
     But it's seems that on boot the mutlipath.conf file is not read.  
Am I right?
When the system is booted, multipathd cannot use the boot device ...
What can I do to fix the boot device name persistence? Where is supposed 
to be configuration file that defines the multipath options of the boot 
device ?

Right now, the server can boot because my fstab is like this:
LABEL=/1                /                      LABEL=/boot             
/boot

Thanks you for your help

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RE: Multipath on boot and root partition with Redhat5.0

[Romanowski, John (OFT)]

I use multipath boot (/boot) and root (/) on SLES9 kernel 2.6.5 on IBM
mainframe Linux with scsi LUNs. I'm not familiar with redhat5 or HP, but
from your post I guess it's newer kernel and newer multipath-tools than
my SLES9.
I did a howto for SLES9
http://www.linuxvm.org/Info/HOWTOs/root-on-multipath.html
but probably won't help you other than hinting at a way; many
differences in our two linux systems.

SLES 10 (kernel 2.6.16?) says it supports root-on-multipath (not boot
though), may be a common feature with redhat 5 too?

As you said, "on boot the multipath.conf file is not read", so that
maybe explains your mpath0 has defaulted alias name when you really
wanted the persistent name to be os_lun as per your multipath.conf. And
the fstab LABEL='s get you around that unexpected mpath0 device name.

General idea is to modify the initial ramfs /init script to run
multipath early; put your current multipath.conf in the initramfs cpio
archive so your aliases are used.  Note that though I have a
multipath-ed /boot after multipath runs, I boot from any single one of
its available paths; mainframe boot loader's not aware of the multiple
paths.
 
good luck


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-----Original Message-----

From: dm-devel-bounces@redhat.com [mailto:dm-devel-bounces@redhat.com]
On Behalf Of Sandrine Twardy
Sent: Wednesday, October 10, 2007 6:59 AM
To: dm-devel@redhat.com
Subject: [dm-devel] Multipath on boot and root partition with Redhat5.0

Hello,

I have a SAN Eva 5000 HP And I have installed a redhat 5.0 on my server 
with "linux mpath", so far all is fine.
I have an os device multipathed as follow :

 > multipath -ll
mpath0 () dm-0 COMPAQ,HSV110 (C)COMPAQ
[size=7.0G][features=0][hwhandler=0]
\_ round-robin 0 [prio=0][active]
\_ 3:0:0:1 sdb 8:16  [active][ready]
\_ 4:0:0:1 sdc 8:32  [active][ready]

But when I presents a new lun to my host and scans again the scsi lun, I

am losing the multipath on the disk (/ and /boot)
(possibly because the name of device is changed from sdb/sdc to sdc/sde
? )

Here's an extract of my multipath.conf:

multipath {
              wwid                    3600508b40000c1c50001c000000b0000
              path_grouping_policy    group_by_serial
              rr_weight               uniform
              prio_callout            /bin/true
              alias                   os_lun
      }
     But it's seems that on boot the mutlipath.conf file is not read.  
Am I right?
When the system is booted, multipathd cannot use the boot device ...
What can I do to fix the boot device name persistence? Where is supposed

to be configuration file that defines the multipath options of the boot 
device ?

Right now, the server can boot because my fstab is like this:
LABEL=/1                /                      LABEL=/boot             
/boot

Thanks you for your help

Re: Multipath on boot and root partition with Redhat5.0

[Hannes Reinecke]

Romanowski, John (OFT) wrote:
> I use multipath boot (/boot) and root (/) on SLES9 kernel 2.6.5 on IBM
> mainframe Linux with scsi LUNs. I'm not familiar with redhat5 or HP, but
> from your post I guess it's newer kernel and newer multipath-tools than
> my SLES9.
> I did a howto for SLES9
> http://www.linuxvm.org/Info/HOWTOs/root-on-multipath.html
> but probably won't help you other than hinting at a way; many
> differences in our two linux systems.
> 
Yeah, I know. Thanks for the script btw.
Yes, in principle we can do root on multipath for SLES9. However,
as YaST is not aware of udev in SLES9 we cannot say we support it.
Sad, but true.

> SLES 10 (kernel 2.6.16?) says it supports root-on-multipath (not boot
> though), may be a common feature with redhat 5 too?
> 

(I knew people would get this wrong. They always do. Sigh)

_Booting_ as something which is provided by the BIOS. And as I haven't
seen _any_ BIOS yet which allows for truly multipathed booting we
_cannot_ boot from multipath. The kernel and the initrd will _always_
be loaded from a single path by the BIOS. So in that sense we don't
support boot from multipath as the BIOS doesn't support it.
Once the kernel and the initrd is loaded into memory then of course
we'll be doing multipathing.
Right from the start. With udev and everything.

And please file a bugzilla or drop me a note if it doesn't work.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		      zSeries & Storage
hare@suse.de			      +49 911 74053 688
SUSE LINUX Products GmbH, Maxfeldstr. 5, 90409 Nürnberg
GF: Markus Rex, HRB 16746 (AG Nürnberg)

Re: Multipath on boot and root partition with Redhat5.0

[Benjamin Marzinski]

On Wed, Oct 10, 2007 at 12:58:41PM +0200, Sandrine Twardy wrote:
> Hello,
> 
> I have a SAN Eva 5000 HP And I have installed a redhat 5.0 on my server 
> with "linux mpath", so far all is fine.
> I have an os device multipathed as follow :
> 
> > multipath -ll
> mpath0 () dm-0 COMPAQ,HSV110 (C)COMPAQ
> [size=7.0G][features=0][hwhandler=0]
> \_ round-robin 0 [prio=0][active]
> \_ 3:0:0:1 sdb 8:16  [active][ready]
> \_ 4:0:0:1 sdc 8:32  [active][ready]
> 
> But when I presents a new lun to my host and scans again the scsi lun, I 
> am losing the multipath on the disk (/ and /boot)
> (possibly because the name of device is changed from sdb/sdc to sdc/sde ? )
> 
> Here's an extract of my multipath.conf:
> 
> multipath {
>              wwid                    3600508b40000c1c50001c000000b0000
>              path_grouping_policy    group_by_serial
>              rr_weight               uniform
>              prio_callout            /bin/true
>              alias                   os_lun
>      }
>     But it's seems that on boot the mutlipath.conf file is not read.  
> Am I right?
> When the system is booted, multipathd cannot use the boot device ...
> What can I do to fix the boot device name persistence? Where is supposed 
> to be configuration file that defines the multipath options of the boot 
> device ?
> 
> Right now, the server can boot because my fstab is like this:
> LABEL=/1                /                      LABEL=/boot             
> /boot
> 
> Thanks you for your help

Multipathed root support in kind of rough in RHEL5.0.  It is much nicer in
5.1.  When you say "But when I presents a new lun to my host and scans again
the scsi lun, I am losing the multipath on the disk" do you mean that if
you add the lun, and rescan without rebooting, that you lose your root
filesystem?  If so, that's strange.  Multipath shouldn't remove the device if
it is currently being used.

Assuming that the problem happens on reboot, the best way to deal with it is to
get your multipaths set up the way you want, and remake the initrd with
mkinitrd.  You should save a backup of your old initrd, so that if there is a
problem with the new initrd, you can still boot your system with the old one.
You might even want to have a seperate grub.conf entry for each initrd.

-Ben

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