Space calculation for ASM

[Salman]

Hi,
Mr. Tim, I am having some serious problems while calculating the usable space in ASM diskgroups because if I calculate personally for a diskgroup space usage for a high or normal redundancy diskgroups, I have no idea where ASM is wasting the disk.
Currently I am testing it on VMWare with 11.1.0.7 running on Linux. Same issue is reproducible in any platform.

Question: If I have to store 2G data on a normal redundancy diskgroup, how much storage should I buy?

If I get 4 disks with 1G size each and create 2 failure groups by putting 2 disks in each failure group, theoretically I should be able to have 2G usable space, but see bellow the result

Code: Select all

create diskgroup test normal redundancy
failgroup d1 disk '/dev/raw/raw3','/dev/raw/raw4'
failgroup d2 disk '/dev/raw/raw5','/dev/raw/raw6';


in ASM, check the actual usable space which I have

Code: Select all

ASMCMD> lsdg test

State    Type    Rebal    Sector    Block    AU     Total_MB Free_MB Req_mir_free_MB Usable_file_MB Offline_disks    Name
MOUNTED NORMAL    N          512    4096    1048576 4076     3970     1019            1475             0       TEST/


Why I am having 1475 only as usable. Oracle gives formula to calculate value in "Usable_file_MB" but this formula is only applicable after creating diskgroup. How can I calculate that how much storage should I buy to store "N" GB of data on a normal redundancy diskgroup.?

Salman

[Tim...]

Hi.

If you check out the docs you will see what this means.

http://docs.oracle.com/cd/B28359_01/ser ... m#BABHHCBH

Free is the total amount of free disk space in the disc group, irrespective of mirroring. Usable is the amount of free space, taking mirroring into account. Some examples might help.

Code: Select all

Normal Redundancy (2-way mirroring)
Free=6000
Usable=3000

High Redundancy (3-way mirroring)
Free=6000
Usable=2000

Cheers

Tim...

[Salman]

Thanks for answering Tim,
Here is actually the problem. Result is not same what you showed in example.
You said

Normal Redundancy (2-way mirroring)
Free=6000
Usable=3000

But actually usable is not 3000, please see bellow

Code: Select all

SQL> create diskgroup test normal redundancy
failgroup d1 disk '/dev/raw/raw3','/dev/raw/raw4','/dev/raw/raw5'
failgroup d2 disk '/dev/raw/raw6','/dev/raw/raw7','/dev/raw/raw8'; 

Diskgroup created.

ASMCMD> lsdg test
State    Type    Rebal  Sector  Block       AU  Total_MB  Free_MB  Req_mir_free_MB  Usable_file_MB  Offline_disks  Name
MOUNTED  NORMAL  N         512   4096  1048576      6114     6004             1019            2492              0  TEST/



You said
High Redundancy (3-way mirroring)
Free=6000
Usable=2000

But usable is not 2000

Code: Select all

SQL> create diskgroup test high redundancy
failgroup d1 disk '/dev/raw/raw3','/dev/raw/raw4'
failgroup d2 disk '/dev/raw/raw5','/dev/raw/raw6'
failgroup d3 disk '/dev/raw/raw7','/dev/raw/raw8';

Diskgroup created.

ASMCMD> lsdg test
State    Type  Rebal  Sector  Block       AU  Total_MB  Free_MB  Req_mir_free_MB  Usable_file_MB  Offline_disks  Name
MOUNTED  HIGH  N         512   4096  1048576      6114     5955             2038            1305              0  TEST/



So there is quite a difference between what we calculate and what ASM provides us. So I am actually looking for some forumla because when we talk about tera bytes of data, the actual usable will be even more less in giga bytes.

Salman

[Tim...]

Hi.

Yes. I was being very simplistic. I'm not sure of an exact calculation to work this out. Maybe you need to speak to Oracle support if you need this level of detail.

Cheers

Tim...