ASM disks and how disks are balanced.
Date: Tue, 15 Nov 2011 11:44:38 +0000
Message-ID: <EC65ECF8123FEE4D8FC5B212637C30400118E05829E1_at_EXCH1.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,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, 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 DATAMRDW 8 10464 15536 11654 NAME MIN MAX AVG
------------------------------ ---------- ---------- ------
DATA 47056 50344 48131 -- 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
It is noticeable that the FRA disks for each database are evenly spaced with the min/max/avg all being closer together whereas DATA shows much more difference. This is probably because the archivelogs and flashbacks logs are of a uniform size.
Rebalancing makes no difference. As can be seen from the last 2 examples.
To make it clear I show a disk listing from the first database I listed above. That is not an unusual situation from what I can see but it does not fit in with 'evenly distributed' as per the documentation.
1 0 MEMBER ONLINE NORMAL UNKNOWN 51200 8400 DATA_0000 DATA_0000
1 1 MEMBER ONLINE NORMAL UNKNOWN 51200 8364 DATA_0001 DATA_0001
1 2 MEMBER ONLINE NORMAL UNKNOWN 51200 8595 DATA_0002 DATA_0002
1 3 MEMBER ONLINE NORMAL UNKNOWN 51200 8434 DATA_0003 DATA_0003
1 4 MEMBER ONLINE NORMAL UNKNOWN 51200 9204 DATA_0004 DATA_0004
1 5 MEMBER ONLINE NORMAL UNKNOWN 51200 9158 DATA_0005 DATA_0005
1 6 MEMBER ONLINE NORMAL UNKNOWN 51200 9826 DATA_0006 DATA_0006
1 7 MEMBER ONLINE NORMAL UNKNOWN 51200 9863 DATA_0007 DATA_0007
1 8 MEMBER ONLINE NORMAL UNKNOWN 51200 9601 DATA_0008 DATA_0008
1 9 MEMBER ONLINE NORMAL UNKNOWN 51200 9559 DATA_0009 DATA_0009
1 10 MEMBER ONLINE NORMAL UNKNOWN 51200 11323 DATA_0010 DATA_0010 XXXXXXX
1 11 MEMBER ONLINE NORMAL UNKNOWN 51200 14258 DATA_0011 DATA_0011 XXXXXXX
1 13 MEMBER ONLINE NORMAL UNKNOWN 51200 8736 DATA_0013 DATA_0013
1 14 MEMBER ONLINE NORMAL UNKNOWN 51200 8606 DATA_0014 DATA_0014
1 15 MEMBER ONLINE NORMAL UNKNOWN 51200 8389 DATA_0015 DATA_0015
1 16 MEMBER ONLINE NORMAL UNKNOWN 51200 8661 DATA_0016 DATA_0016
1 17 MEMBER ONLINE NORMAL UNKNOWN 51200 8428 DATA_0017 DATA_0017
1 18 MEMBER ONLINE NORMAL UNKNOWN 51200 8282 DATA_0018 DATA_0018
1 19 MEMBER ONLINE NORMAL UNKNOWN 51200 8546 DATA_0019 DATA_0019
1 20 MEMBER ONLINE NORMAL UNKNOWN 51200 8426 DATA_0020 DATA_0020
1 21 MEMBER ONLINE NORMAL UNKNOWN 51200 8444 DATA_0021 DATA_0021
1 22 MEMBER ONLINE NORMAL UNKNOWN 51200 8628 DATA_0022 DATA_0022
So my question is, is this normal and expected (which I think it is based on my systems -all 11GR1, external redundancy ) or should we be seeing better disk balance than we are doing.
John
www.jhdba.wordpress.com
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http://www.freelists.org/webpage/oracle-l Received on Tue Nov 15 2011 - 05:44:38 CST