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regarding to required_mirror_free_mb [message #580199] Thu, 21 March 2013 04:55 Go to next message
lonion
Messages: 97
Registered: August 2011
Location: shenzhen,China
Member

In V$ASM_DISKGROUP have a column named required_mirror_free_mb.

My question is :
1. what's required mirror ?
2. why asm reserve mirror space ?
3. how Oracle Automatic Storage Management (ASM) calculates the REQUIRED_MIRROR_FREE_MB ?
Re: regarding to required_mirror_free_mb [message #580200 is a reply to message #580199] Thu, 21 March 2013 05:03 Go to previous messageGo to next message
John Watson
Messages: 4404
Registered: January 2010
Location: Global Village
Senior Member
The calculation is derived from the size of your failgroups (by default, each disc is a failgroup) and your redundancy settings. It represents the space needed to re-establish mirrors if a device is lost.
Re: regarding to required_mirror_free_mb [message #580228 is a reply to message #580200] Thu, 21 March 2013 10:57 Go to previous message
lonion
Messages: 97
Registered: August 2011
Location: shenzhen,China
Member
From this article : http://joordsblog.vandenoord.eu/2011/05/asm-required-mirror-free-space-to.html

it said that REQUIRED_MIRROR_FREE_MB depends on the redundancy level and the number of failure groups in the disk group. like the following:
● Normal redundancy disk groups with at least three failure groups require an amount of free space that is equal to the size of a single failure group. Normal redundancy disk groups with only two failure groups require an amount of free space that is equal to the size of a single disk.

● High redundancy disk groups with at least four failure groups require an amount of free space that is equal to the sum of the size of two failure groups. High redundancy disk groups with only three failure groups require an amount of free space that is equal to the sum of the size of two disks.

i'm so confused about the above conclusion , who can explain it for me clearly ?

why when it is Normal redundancy disk groups with at least three failure groups , REQUIRED_MIRROR_FREE_MB is equal to the size of a single failure group , not the size of 2 failure groups.
why when it is Normal redundancy disk groups with only two failure groups ,REQUIRED_MIRROR_FREE_MB is equal to the size of a single disk. , not the size of 2 disks or 3 disks and so on .



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