Re: ASM question

  • From: "Alex Gorbachev" <gorbyx@xxxxxxxxx>
  • To: greg@xxxxxxxxxxxxxxxxxx
  • Date: Sat, 24 Feb 2007 18:25:31 -0500

This math seems a bit odd. It applies to disk group of 4 disks without
separation in two failure groups where by default each disk forms its
own failure group.

How it works in this case:
if one disk fails than Oracle can mirror extents in the following ways:
half of disk1 + half of disk2 = 2.5 GB
+
half of disk2 + half of disk3 = 2.5 GB
+
half of disk1 + half of disk3 = 2.5 GB
=
7.5 GB

However, with disk1 and disk2 being in the same failure groups, Oracle
won't be able to mirror extents between them (first 2.5 GB above) so
it should really be 5 GB.

To the original poster - be sure you know why you want to separate
disks into failure groups. It doesn't make sense if they disks of the
same SAN box, for example. Unless they are accessed by different
controller/FC switch or something.

Chances are 5 GB volume is not exactly one spindle behind. According
to the path - it seems they are volumes from the same Veritas
diskgroup. Though, it's possible to allocate them from particular
disk(s), that's probably not the case. Is it? So it hardly justifiable
to split them in such small chunks.

Since you already using Veritas, you might as well go for their
mirroring instead of ASM normal redundancy as more mature solution.

On 2/23/07, Greg Rahn <greg@xxxxxxxxxxxxxxxxxx> wrote:


 To benefit the list...
 --
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

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Best regards,
Alex Gorbachev

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