Re: ASM and EMC PowerPath

  • From: "Radoulov, Dimitre" <cichomitiko@xxxxxxxxx>
  • To: "Leyi Zhang (Kamus)" <kamusis@xxxxxxxxx>
  • Date: Sun, 10 Apr 2011 12:12:50 +0200


OK,
as in the MOS note Martin suggested and the others have already mentioned,
in order for asmlib to use multipathing we need to set scanexclude
or/and scanorder. So even querydisk -p reports all available paths
to a given disk, it uses the first one it finds.

If we add new disks using the correct path (emcpower...)
and we don't set the above mentioned asm configuration variables,
after scanning the disks on the other node, asm will pick the first
path found, not the one we used with the createdisk command.
I don't know if this can cause problems.

So I would first add disks (because this is urgent)
using the first devices in ASCIIbetical order and asm will/should
pick the same devices on both nodes (at least I hope so).

After that, when downtime is possible (or better yet, if a test environment
could be build) I'll try to fix the paths (use the emcpower paths).


Best regards
Dimitre


On 10/04/2011 11:50, Radoulov, Dimitre wrote:


So the first question would be:
what would happen if the path reported by querydisk -d for a given disk is not available?
Would we get an error? Would asm switch to some of the other available paths to the same disk automatically?
If asmlib could handle this, the choice of the path during disk creation should be irrelevant. 



[...]



On 10/04/2011 11:13, Radoulov, Dimitre wrote:


Hi Leyi,
answers inline.


What my point is , since you can get this result:

# /etc/init.d/oracleasm querydisk -p BLOB
Disk "BLOB" is a valid ASM disk
/dev/sdam1: LABEL="BLOB" TYPE="oracleasm"
/dev/sdbc1: LABEL="BLOB" TYPE="oracleasm"
/dev/emcpowerz1: LABEL="BLOB" TYPE="oracleasm"

Then it means emcpower device was used when creatediesk.  Actually, no
matter what device name was used when createdisk, at least now your
"/dev/emcpowerz1" is recognized as ASM Disk BLOB. it's enough (If I'm
wrong please correct me).


Actually, that's what my original email is all about.

I see that the emcpower devices are recognized as asm disks,
I don't know if this is enough though ...

I suppose that querydisk just reports all devices/paths that point
to the physical disk. What I want to know is if asm is smart
enough to pick the emcpower device in this case or it
just uses the first one found (in ASCIIbetical order).

querydisk -d returns the major/minor of one of the physical paths
to the disk, not the emcpower path (I checked /proc/partitions for that), 
that's why I think it actually doesn't use the logical devices.
I say "I think", because I'm not finding this anywhere in the docs for now. 


Query disk typically is safe, you can modify "ORACLEASM_SCANORDER" and
"ORACLEASM_SCANEXCLUDE" when you get some downtime in database
maintenance window then restart ASM, if disk scanned is not an ASM
disk, will just simply raise an error, no data broken. (Of course,
backup your database first)


Thanks for the suggestion!
Yes, most likely we'll try this first.


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