[Linux-cluster] LVM and Multipath with EMC PowerPath (Was: CLVMD - Do I need it)

[Celso K. Webber]

Hello all,

After reading a thread on this list (CLVMD - Do I need it), I started 
playing around with CLVM, just to make sure two problems I had in the 
past were solved:

1) LVM normally cannot be used on shared disks, because the first server 
that "sees" the PVs will initialize them, and the other server will see 
the LVM objects as inactive. This is solved in LVM2 when used together 
with CLVM, right? I'm not pretty sure about the mecanics of CLVM, but I 
imagine it shares device UUIDs between the machines. So far, so good.

2) The other problem is not directly related to CLVM, but I found no 
solution for it (yet). In my setup, I have multiple paths to the same 
devices in the shared storage (either in a SAN or DAS). Under the EMC 
solution, we employ PowerPath to solve the multiple devices issue for 
each LUN. It works quite well. But LVM is not aware of PowerPath's 
multiple path aggregation, so when it scans the PVs on the LUN's 
partitions, it "finds" duplicates for the PVs, like this:
[root at csumccaixa12 network-scripts]# pvscan
   Found duplicate PV 7v9XUzPHIRqe6E0fA6hgCR3ybeaJoiWm: using /dev/sdc1 
not /dev/emcpowerb1
   Found duplicate PV 3eKnMIm00kg6DXn4MW1UX9QCFh96ykwG: using 
/dev/emcpowerc1 not /dev/sdb1
   Found duplicate PV 3T00PR5Ky1XrBesYHRtyowoBQLWDO1kd: using /dev/sdd1 
not /dev/emcpowera1
   Found duplicate PV 3eKnMIm00kg6DXn4MW1UX9QCFh96ykwG: using /dev/sde1 
not /dev/emcpowerc1
   Found duplicate PV 7v9XUzPHIRqe6E0fA6hgCR3ybeaJoiWm: using /dev/sdf1 
not /dev/sdc1
   Found duplicate PV 3T00PR5Ky1XrBesYHRtyowoBQLWDO1kd: using /dev/sdg1 
not /dev/sdd1
   PV /dev/sda3   VG vg0   lvm2 [59.81 GB / 37.75 GB free]
   PV /dev/sdg1            lvm2 [127.43 GB]
   PV /dev/sde1            lvm2 [127.43 GB]
   PV /dev/sdf1            lvm2 [127.43 GB]
   Total: 4 [442.10 GB] / in use: 1 [59.81 GB] / in no VG: 3 [382.29 GB]

You can see above that the /dev/emcpowerX devices were declined in favor 
of the real Linux devices. "vg0" is a VG in the internal disks (/dev/sda).

The problem I see here is that whenever the specific device that LVM2 
chose goes down because of a link failure, LVM will not automatically 
failover to another device, will it? In my tests it didn't.

Another matter is that using the /dev/emcpowerX devices I have also load 
balancing, so even if LVM2 did failover to the other paths (the other 
devices), I would loose the load balancing feature I can achieve with 
PowerPath.


Question 1: did anyone solve this problem? Does device-mapper-multipath 
solve this problem?

Question 2: is there a way to "force" which devices LVM should employ 
when scanning the PVs over the disks Linux recognize?


Thank you all for any hints on this.

Regards,

Celso.
-- 
*Celso Kopp Webber*

celso at webbertek.com.br <mailto:celso at webbertek.com.br>

*Webbertek - Opensource Knowledge*
(41) 8813-1919
(41) 3284-3035


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