RE: ASM disks and how disks are balanced.

From: John Hallas <John.Hallas_at_morrisonsplc.co.uk>
Date: Tue, 15 Nov 2011 15:40:30 +0000
Message-ID: <EC65ECF8123FEE4D8FC5B212637C30400118E0582C9D_at_EXCH1.morrisonsplc.co.uk>

Thanks Suse.
This is a quiet test system so the rebalance level was OK at 8. I agree that it is not the most important thing in the world but I am only challenging the documentation that states that disks are rebalanced evenly. I can see that they are with FRA diskgroups but not with DATA ones. I am working on Dave's suggestion of multiple consecutive rebalances and I can see it makes a subtle difference (there are about 10 rebalance commands between the 3 snapshots shown below) but not a massive difference.

NAME                               AU(Mb)        MIN        MAX        AVG


------------------------------ ---------- ---------- ---------- ----------

DATA                                    8      48528      51496 50278.1176
DATA                                    8      48560      51496 50278.1176
DATA                                    8      48912      51232 50278.1176

I have also looked at the imbalance between files and I cannot see anything difference or improvement after doing the imbalances. Unfortunately the biggest difference I can see is on a production database diskgroup and I cannot experiment with that.

John

From: Suse Shi [mailto:sinosuse_at_gmail.com] Sent: 15 November 2011 14:48
To: John Hallas
Cc: Oracle-L Group
Subject: Re: ASM disks and how disks are balanced.

Hi John,

   from your query, we can see the disks space in a very similar scope.

   I think it's expected and no worry at the difference on data, ASM instance will try to do rebalance jobs and keep space-usage 'SAME' on different disks, but it doesn't mean that ASM diskgroup will do rebalance once there're tiny modifications, that should be a waste of performance.

   If you're working on a test environment, just do add/drop disks, add/drop large files jobs and see the changes, I think the difference will always be in an acceptable scope.

   btw, if your systems are in busy transactions state, please don't set power value so high as '8', it will also decrease the database i/o performance.

  wish helps,

-S

2011/11/15 John Hallas <John.Hallas_at_morrisonsplc.co.uk<mailto:John.Hallas_at_morrisonsplc.co.uk>>

Looking at a variety of systems I can see that there is a difference between the space available on each disk of a ASM disk group. This is despite the documentation stating :-

If the disks are the same size, then ASM spreads the files evenly across all of the disks in the disk group. This allocation pattern maintains every disk at the same capacity level and ensures that all of the disks in a disk group have the same I/O load. Because ASM load balances among all of the disks in a disk group, different ASM disks should not share the same physical drive.

The following query shows a selection of databases and how the disks are striped. In all examples the disks in the disk group are the same size - normally 100Gb

Select dg.name<http://dg.name>,dg.allocation_unit_size/1024/1024 "AU(Mb)",min(d.free_mb) Min, max(d.free_mb) Max, avg(d.free_mb) Avg   from v$asm_disk d, v$asm_diskgroup dg
   where d.group_number = dg.group_number    group by dg.name<http://dg.name>, dg.allocation_unit_size/1024/1024 /

NAME                               AU(Mb)        MIN        MAX    AVG


------------------------------ ---------- ---------- ---------- ------

FRA                                     1      11364      11470  11390
DATA                                    1       8282      14258   9170

NAME                               AU(Mb)        MIN        MAX    AVG


------------------------------ ---------- ---------- ---------- ------

FRA                                     1      72659      72920  72837
DATA                                    1      19377      30987  22157<tel:30987%C2%A0%2022157>
DATAMRDW                                8      10464      15536  11654


NAME                                  MIN        MAX    AVG


------------------------------ ---------- ---------- ------

DATA                                47056      50344  48131<tel:50344%C2%A0%2048131> -- before rebalance
FRA                                 22740      22769  22752

alter diskgroup data rebalance power 8;

NAME                               AU(Mb)        MIN        MAX    AVG


------------------------------ ---------- ---------- ---------- ------

FRA                                     1      22740      22769  22752
DATA                                    8      47824      49096  48131 -- after rebalance  -  no real difference


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http://www.freelists.org/webpage/oracle-l Received on Tue Nov 15 2011 - 09:40:30 CST