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@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@xxxxxxxxxxxxxxxx <mailto:celso@xxxxxxxxxxxxxxxx>
*Webbertek - Opensource Knowledge*
(41) 8813-1919
(41) 3284-3035
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