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Re: [dm-devel] Setting up a multipath device



Christophe Varoqui wrote:

On Thu, May 04, 2006 at 05:08:41PM +0300, Erez Zilber wrote:
Hi,

I'm new to device mapper and trying to set a multipath target:

I'm connected to the same LUN over iSCSI in 2 paths:

Here's the output of sg_map:
sg_map -i -x
/dev/sg0 3 0 0 0 0 /dev/sda DotHill SANnet II FC 411I
/dev/sg1 4 0 0 0 0 /dev/sdb DotHill SANnet II FC 411I

I ran the following command:
multipath 2 round-robin 1 0 /dev/sda round-robin 1 0 /dev/sdb

Just start multipathd
or run multipath without params

If I understand correctly, the 1st priority group contains /dev/sda. The 2nd priority group contains /dev/sdb. If /dev/sda fails, traffic should automatically move to /dev/sdb. Is that right?

Here's the out put of multipath -l:

3600c0ff00000000007c1d121c397f503
[size=10 GB][features=0][hwhandler=0]
\_ round-robin 0 [prio=0][active]
\_ 3:0:0:0 sda 8:0 [active][undef]
\_ round-robin 0 [prio=0][enabled]
\_ 4:0:0:0 sdb 8:16 [active][undef]

What does 8:0 & 8:16 mean? What is 3600c0ff00000000007c1d121c397f503?

8:0 & 8:16 are paths major:minor numbers
3600c0ff00000000007c1d121c397f503 is the multipath name.
consider activating the fancy naming option in the config file.

Now, assuming that what I did so far is ok, how do I use the multipath device? Do I use /dev/sda (or /dev/sdb)?

You should use /dev/mapper/3600c0ff00000000007c1d121c397f503

Regards,
cvaroqui

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Thanks. It seems to work now. Anyway, I have some more questions:

  1. Was it wrong to run 'multipath 2 round-robin 1 0 /dev/sda
     round-robin 1 0 /dev/sdb'?
  2. If I don't supply any parameters, how does the dm know which
     devices are the same physical device (that /dev/sda & /dev/sdb are
     the same physical device)?
  3. How are the priority groups defined if no parameters are supplied?
  4. What is the timeout for a device that has failed? When does the dm
     move to the next priority group?


Thanks
--

____________________________________________________________

Erez Zilber | 972-9-971-7689

Software Engineer, Storage Team

Voltaire – _The Grid Backbone_

__

www.voltaire.com <http://www.voltaire.com/>



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