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7 Replies Latest reply: Sep 11, 2012 7:58 PM by salmanqureshi-2314430

# Space usage in ASM

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Hi,
I have understanding o ASM, Diskgroups and Failuregroups in 11.2. I have question regarding how space is used/allocated in ASM. I have 5 didks (120GB each) for which I want to create a high redundancy disk group. Since ASM will do 3 way mirroring by keeping 3 copies of each data extent. Since this is extent level mirroring, how can calculate the usable space from total 500GB? Can we say 500/3=166 Usable space? Do we calculate space like this in 3 way mirroring using high redundancy?
Thanks
• ###### 1. Re: Space usage in ASM
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Hi,

5 x 120GB will roughly get 600GB. With high redundancy the usable space would be roughly 200GB (600GB/3)

Hope it helps

Cheers
• ###### 2. Re: Space usage in ASM
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. Since ASM will do 3 way mirroring by keeping 3 copies of each data extent.
all depend on your configuration ,external redundancy required no mirroring.
normal - 2 way and high -3 way mirroring.

suppose you've 120GB of 3 disk then total usable space would be 120GB as other two required for mirroring.
and in future you want to increase it by 120GB more then again you need 120GB of 3 disk.
• ###### 3. Re: Space usage in ASM
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If you do high redundancy, you need 3 disks of the same size to get the usable space = size of one of them, so if you have 5 disks, you won't be able to add 2 of those disks since you will be missing one more disk to have a total of 240Gb on the resulting diskgroup.

5x120G

redundancy normal (2) = 4 disks used = 240G usable - 1 disk left for spare / other DG
redundancy high (3) = 3 disks used = 120Gb usable - 2 disks left for spare or separate DG
• ###### 4. Re: Space usage in ASM
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Same calculation is not working for me. See my high redundancy diskgroup having 4 disks, 73G each
State    Type  Rebal  Sector  Block       AU  Total_MB  Free_MB  Req_mir_free_MB  Usable_file_MB  Offline_disks  Voting_files  Name

MOUNTED  HIGH  N         512   4096  4194304    280016   279612           140008           46534              0             N  REDO/

[code}

With 4 disks of 70G each, total size is 280G roughly and with a high redundancy, should usable space be 280/3=93G roughly?

SAQ
• ###### 5. Re: Space usage in ASM
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post output in { code } (remove space) use preview before posting
and
output of below query
SELECT
name                                     group_name
, sector_size                              sector_size
, block_size                               block_size
, allocation_unit_size                     allocation_unit_size
, state                                    state
, type                                     type
, total_mb                                 total_mb
, (total_mb - free_mb)                     used_mb
, ROUND((1- (free_mb / total_mb))*100, 2)  pct_used
,free_mb
,USABLE_FILE_MB
,OFFLINE_DISKS
FROM
v\$asm_diskgroup
ORDER BY
name
/
• ###### 6. Re: Space usage in ASM
Currently Being Moderated
Hi,

You have 4 disks, each 73G,
If you are create high redundancy disk group with 3 disk then usable space size will be 73G only. Because, this three disk is mirroring - 73G

Regards
• ###### 7. Re: Space usage in ASM
Currently Being Moderated
GROUP_NAME                     SECTOR_SIZE BLOCK_SIZE ALLOCATION_UNIT_SIZE STATE       TYPE     TOTAL_MB    USED_MB   PCT_USED    FREE_MB USABLE_FILE_MB OFFLINE_DISKS
------------------------------ ----------- ---------- -------------------- ----------- ------ ---------- ---------- ---------- ---------- -------------- -------------
DATA                                   512       4096              4194304 MOUNTED     HIGH      4915200       1508        .03    4913692        1474057             0
RECO                                   512       4096              4194304 MOUNTED     HIGH      6416800     315232       4.91    6101568        1819962             0
REDO                                   512       4096              4194304 MOUNTED     HIGH       280016        404        .14     279612          46534             0
Let me put question in even simpler way.
If I have store a database of 1 terabyte size, how much space I should use to create the diskgroup with high redundancy to store my 1 terabyte of data. Should I create a diskgroup with 3 terabyte storage allocated to it? Certainly the USABLE_FREE, FREE_MB and REQUIRED_MIRROR_FREE are the values I can get after creating the diskgroup, but how much to allocate to diskgroup should be know prior to creating the diskgroup.

SAQ

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• Correct Answers - 10 points