Re: ASM question

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_at_structureddata.org> wrote:
>
> To benefit the list...
> --
> Greg Rahn
> http://structureddata.org
>
>
> -------- Original Message --------
> Subject: Re:ASM question
> From: "Hameed, Amir" <Amir.Hameed_at_xerox.com>
> To: "Greg Rahn" <greg_at_structureddata.org>
> Date: 2/23/2007 1:15 PM
>
>
> Thank you for your explanation.
> Amir
>
> ________________________________
> From: Greg Rahn [mailto:greg_at_structureddata.org]
> 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_at_xerox.com>
> To: oracle-l_at_freelists.org
> 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
>
> --
> http://www.freelists.org/webpage/oracle-l
>
>
>
>
>
>

-- 
Best regards,
Alex Gorbachev

http://www.oracloid.com
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