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ASM disk inbalanced

Questions relating to Oracle Real Application Clusters (RAC) and Clusterware.

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ASM disk inbalanced

Postby sivakumarocp » Tue Jan 15, 2013 4:59 am

Hi Tim,

I would like to know, when we can say ASM disks are imbalanced.
Either disks in different size with in ASM diskgroup OR when we have different size ASM diskgroup (with unique disk size in each diskgroup).

Currently one of our Production database (standalone DB) running in ASM storage in that we have multiple diskgroups.

each diskgroup having multiple disks.

Code: Select all

Group# NAME               Size_GB
------ ------------------ -------
     1 ARCHLOG                200
     3 DATAFILE               250
     2 DATAFILE_ORAPRD88      375
     6 DATAFILE_ORAPRD86     1000
     4 LOG1                    20
     5 LOG2                    20

6 rows selected.

    Group# PATH                          Size_GB
---------- ----------------------------- -------
         1 /dev/oracleasm/disks/ARCH003      100
         1 /dev/oracleasm/disks/ARCH002      100
         2 /dev/oracleasm/disks/DATA004      125
         2 /dev/oracleasm/disks/DATA003      125
         2 /dev/oracleasm/disks/DATA005      125
         3 /dev/oracleasm/disks/DATA002      125
         3 /dev/oracleasm/disks/DATA001      125
         4 /dev/oracleasm/disks/LOG011        10
         4 /dev/oracleasm/disks/LOG003        10
         5 /dev/oracleasm/disks/LOG012        10
         5 /dev/oracleasm/disks/LOG014        10
         6 /dev/oracleasm/disks/DATA009      250
         6 /dev/oracleasm/disks/DATA008      250
         6 /dev/oracleasm/disks/DATA007      250
         6 /dev/oracleasm/disks/DATA006      250

15 rows selected.

As you can see, DATAFILE_ORAPRD88 diskgruop has (3*125G) 3 disks and DATAFILE_ORAPRD86 has 4 disks (4*250),
Three instances are running under DATAFILE_ORAPRD88 diskgruop, and those DB's are very high OLTP are happening.

Last week one of our L4 DBA reported that heavy I/O are happening on this host, especially DATAFILE_ORAPRD88 diskgroup.

finally they suggested like, all disks size in a ASM diskgroup should be of same size, hence he asked us to request more LUNs with 100G each.
and add it to existing diskgroup and asked us to remove those old disks to balance I/O. This is what he is suggested.

As per my knowledge, Oracle recommends ASM diskgroup should have equally sized disks to get I/O balanced!. am I right here?

2. Unix Admin reported that DATAFILE_ORAPRD88 diskgruop having more I/O always, so I took AWR report on all three DBs which is accessing
this diskgroup, and found two DBs are causing huge Physical I/O. upon checking further found same query is running on both the database and its been missing an Index.

what I need to know here is, If I create an Index for that query, I am sure I/O will reduce, if so does I/O on DATAFILE_ORAPRD88 diskgroup will reduce?

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Re: ASM disk inbalanced

Postby Tim... » Tue Jan 15, 2013 8:21 am


1) Read the first 4 bullet points in this section of the article. ... isk_groups

2) Anything that reduces the amount of physical reads and writes in the database will reduce the load on the ASM disks.


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