Ferhat's Blog

There will be only one database

What happens when exadata has lost two disks?

Posted by fsengonul on March 8, 2011

Actually the question is “What happens when ASM has lost two disks in different failgroups when redundancy is normal?”
We’re sure that there will be no problem if we lose all of the disks in a single failgroup but in order to answer the question we have to understand how mirroring works in ASM.
In normal redundancy, mirror of each extent is written to another disk in another failgroup. This causes a partnership between these disks. As far as I’ve found out this information is kept in the x$kfdpartner table.

From the following query it seems that an extend in DATA_CD_10_CEL27 has a mirror on DATA_CD_07_CEL01. If we lose these two disks then the system has to crash.
On the other hand if we lose DATA_CD_10_CEL27 and the disks in the partner list ( 8 disks at the moment) stays alive, there will be no problem if we lose another disk.


select dg.name dg_name,
d.name disk_name,
pd.name partner_name
from x$kfdpartner p,v$asm_diskgroup dg,v$asm_disk d,v$asm_disk pd
where
p.grp=dg.group_number
and d.disk_number=p.disk and d.group_number=p.grp
and pd.disk_number=p.number_kfdpartner and pd.group_number=p.grp
and dg.name='DATA' and d.name='DATA_CD_10_CEL27';

DG_NAME DISK_NAME PARTNER_NAME
DATA DATA_CD_10_CEL27 DATA_CD_07_CEL01
DATA DATA_CD_10_CEL27 DATA_CD_06_CEL01
DATA DATA_CD_10_CEL27 DATA_CD_07_CEL25
DATA DATA_CD_10_CEL27 DATA_CD_06_CEL25
DATA DATA_CD_10_CEL27 DATA_CD_07_CEL26
DATA DATA_CD_10_CEL27 DATA_CD_06_CEL26
DATA DATA_CD_10_CEL27 DATA_CD_07_CEL28
DATA DATA_CD_10_CEL27 DATA_CD_06_CEL28

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