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Greg Rahn
http://structureddata.org
-------- Original Message --------
Subject: Re:ASM question
From: "Hameed, Amir" <Amir.Hameed@xxxxxxxxx>
To: "Greg Rahn" <greg@xxxxxxxxxxxxxxxxxx>
Date: 2/23/2007 1:15 PM
Thank you for your explanation.
Amir
________________________________
From: Greg Rahn [mailto:greg@xxxxxxxxxxxxxxxxxx]
Sent: Friday, February 23, 2007 3:17 PM
To: Hameed, Amir
Subject: Re: ASM question
First, lets understand a couple things:
ASM Failgroups & normal redundancy: Any ASM disk in a given
failgroup may
not have its extent mirrors on any other asm disk in that same failgroup.
A normal redundancy disk group can tolerate the failure of one failure
group. If only one failure group fails, the disk group remains mounted
and
serviceable, and ASM performs a rebalance of the surviving disks
(including
the surviving disks in the failed failure group) to restore redundancy
for
the data in the failed disks. If more than one failure group fails, ASM
dismounts the disk group.
REQUIRED_MIRROR_FREE_MB indicates the amount of space that must be
available in the disk group to restore full redundancy after the worst
failure that can be tolerated by the disk group.
http://download-east.oracle.com/docs/cd/B19306_01/server.102/b14231/storeman.htm
In your second example the 7627GB USABLE_FILE_MB comes from here:
The worst failure that this disk group could tolerate is 1 ASM disk
failure
(this is where REQUIRED_MIRROR_FREE_MB = 5GB comes from) and still
restore
full redundancy. Given that, 100% of the data and its redundant copy
would
have to reside on 3 asm disks. So if ASM needs to support 4 ASM disks
data
on 3 ASM disks no more than 75% of the capacity could be used. Using
normal
redundancy the math would be:
4 ASM disks @5GB = 20GB = TOTAL_MB
20GB TOTAL_MB / 2 = 10GB (for primary extent mirrors)
3/4 (support 4 disks data on 3) * 10GB = 7.5GB
In the first example REQUIRED_MIRROR_FREE_MB is 0 because that diskgroup
could not sustain a failure and still restore full redundancy.
Regards,
-Greg
-------- Original Message --------
Subject: ASM question
From: "Hameed, Amir" <Amir.Hameed@xxxxxxxxx>
To: oracle-l@xxxxxxxxxxxxx
Date: 2/23/2007 10:13 AM
Hi Folks,
I have a 10.2.0.2 ASM instance running on Solaris 9 with the following
scenario:
1. All raw disk slices are 5GB in size
2. I have created a normal redundancy ASM diskgroup with two failure
groups as shown below:
SQL> create diskgroup data normal redundancy
failgroup failgroup_1
disk '/dev/vx/rdsk/ux016_RAW/volraw_01'
failgroup failgroup_2
disk '/dev/vx/rdsk/ux016_RAW/volraw_02'
/
When I run the sql statement, as shown below, I see the following
output:
SQL> select GROUP_NUMBER GROUP#, NAME, STATE, TOTAL_MB, FREE_MB,
REQUIRED_MIRROR_FREE_MB REQ_MIRR_FREE_MB, USABLE_FILE_MB
from V$ASM_DISKGROUP;
GROUP# NAME STATE TOTAL_MB FREE_MB
REQ_MIRR_FREE_MB USABLE_FILE_MB
---------- -------------- --------------- ---------- ----------
---------------- --------------
1 DATA MOUNTED 10240 10138
0 5069
So, the total size of the DG is 10GB with Usable space of 5GB. Because
the group is mirrored 1:1, the REQ_MIRR_FREE_MB is zero.
3. When I create the same group with two disks in each failover group, I
see an output that I am not able to comprehend:
SQL> create diskgroup data
failgroup failgroup_1
disk
'/dev/vx/rdsk/ux016_RAW/volraw_01',
'/dev/vx/rdsk/ux016_RAW/volraw_02'
failgroup failgroup_2
disk
'/dev/vx/rdsk/ux016_RAW/volraw_03',
'/dev/vx/rdsk/ux016_RAW/volraw_04'
/
SQL> select GROUP_NUMBER GROUP#, NAME, STATE, TOTAL_MB, FREE_MB,
REQUIRED_MIRROR_FREE_MB REQ_MIRR_FREE_MB, USABLE_FILE_MB
from V$ASM_DISKGROUP;
GROUP# NAME STATE TOTAL_MB FREE_MB
REQ_MIRR_FREE_MB USABLE_FILE_MB
---------- -------------- --------------- ---------- ----------
---------------- --------------
1 DATA MOUNTED 20480 20374
5120 7627
I was hoping to see REQ_MIRR_FREE_MB of zero because I have a DG that
contains two failure groups with each group contains two disks. I was
also expecting to see 10GB for the USABLE_FILE_MB.
Can someone please clarify how two interpret these stats.
Thanks
Amir
