Expert Oracle Exadata - Parent Directory

CHAPTER 14 STORAGE LAYOUTDATA_DGCELL01CELL03CELL02Figure 14-3. ASM failure groups CELL01 – CELL03There are three types of redundancy in ASM: External, Normal, and High.External Redundancy: No redundancy is provided by ASM. It is assumed thatthe storage array, usually a SAN, is providing adequate redundancy; in mostcases RAID0, RAID10, or RAID5. This has become the most common methodwhere large storage area networks are used for ASM storage. In the Exadatastorage grid, ASM provides the only mechanism for mirroring. If ExternalRedundancy were used, the loss of a single disk drive would mean acatastrophic loss of all ASM diskgroups using that disk. It also means that eventhe temporary loss of a storage cell (reboot, crash, or the like) would make alldisk groups using storage on the failed cell unavailable for the duration of theoutage.Normal Redundancy: Normal Redundancy maintains two copies of data blocksin separate failure groups. Databases will always attempt to read from theprimary copy of a data block first. Secondary copies are only read when theprimary blocks are unavailable. At least two failure groups are required forNormal Redundancy, but many more than that may be used. For example, anExadata full rack configuration has 14 storage cells, and each storage cellconstitutes a failure group. When data is written to the database, the failuregroup used for the primary copy of a block rotates from failure group to failuregroup in a round-robin fashion. This ensures that disks in all failure groupsparticipate in read operations.High Redundancy: High Redundancy is similar to Normal Redundancy exceptthat three copies of data blocks are maintained in separate failure groups.470