Expert Oracle Exadata - Parent Directory

CHAPTER 14 STORAGE LAYOUTFailure GroupsBefore we talk in more detail about grid disks, let’s take a brief detour and talk about how diskredundancy is handled in the ASM architecture. ASM uses redundant sets of ASM disks called failuregroups to provide mirroring. Traditional RAID0 mirroring maintains a block-for-block duplicate of theoriginal disk. ASM failure groups provide redundancy by assigning ASM disks to failure groups andguaranteeing that the original and mirror copy of a block do not reside within the same failure group. Itis critically important to separate physical disks into separate failure groups. Exadata does this byassigning the grid disks from each storage cell to a separate failure groups. For example, the followinglisting shows the fail groups and grid disks for storage cells 1-3. As the names imply, these fail groupscorrespond to storage cells 1–3. These fail groups were created and named automatically by ASM whenthe grid disks were created.SYS:+ASM2> select failgroup, name from v$asm_disk order by 1,2FAILGROUP NAME----------- ----------------------CELL01 DATA_DG_CD_00_CELL01CELL01 DATA_DG CD_01_CELL01CELL01 DATA_DG CD_02_CELL01CELL01 DATA_DG CD_03_CELL01...CELL02 DATA_DG CD_00_CELL02CELL02 DATA_DG CD_01_CELL02CELL02 DATA_DG CD_02_CELL02CELL02 DATA_DG CD_03_CELL02...CELL03 DATA_DG CD_00_CELL03CELL03 DATA_DG CD_01_CELL03CELL03 DATA_DG CD_02_CELL03CELL03 DATA_DG CD_03_CELL03Figure 14-3 shows the relationship between the DATA_DG disk group and the failure groups, CELL01,CELL02, and CELL03. Note that this does not indicate which level of redundancy is being used, only thatthe DATA_DG disk group has its data allocated across three failure groups.469