Oracle Database 11 g - Online Public Access Catalog

CHAPTER 9 ■ STORAGE MANAGEMENT 389initialization parameter must be greater than or equal to the RDBMS compatibility of all diskgroupsused by the database.As stated earlier, ASM compatibility of a diskgroup can be set to 11.0, while its RDBMScompatibility could be 10.1. This implies that the diskgroup can be managed only by ASM softwareversion 11.0 or higher, while any database software version must be 10.1 or higher. Todetermine the software version and compatibility setting of the database, you can query theV$ASM_CLIENT view, as displayed here:select db_name,status,software_version,compatible_versionfrom v$asm_clientSQL> /DB_NAME STATUS SOFTWARE_ COMPATIBLE_VERSION VERSION---------- ------------ ------------- ------------DBA11g1 CONNECTED 11.1.0.6.0 11.1.0.0.0DBA11g1 CONNECTED 11.1.0.6.0 11.1.0.0.0Fast Mirror ResyncIn Oracle Database 10g, ASM offlines a disk when it is not able to write an extent or access thedisk and is not needed anymore, shortly after the disk is dropped from the diskgroup. At thispoint, ASM will perform a rebalance on the extents of the surviving disk members in the diskgroupusing the mirror extent copies. This rebalance operation is extremely costly and can takehours. Even for nondisk problems such as bad cables or problems with HBA or controllers, disksmay get dropped from the diskgroup, and rebalance activity may occur. ASM is ignorant of what iscausing the issue. ASM just knows that it is not able to complete a write operation.In Oracle Database 11g, Oracle assumes the content of a dropped disk is not damaged ormodified and preserves its membership in the diskgroup. Oracle introduces a new feature calledASM fast disk resync and does not automatically drop a disk from the diskgroup for a writefailure. When a disk goes offline, Oracle now tracks all modified extents for a specified durationand keeps the disk membership in the diskgroup intact.■Note The tracking mechanism is similar to a bitmap index. It allocates 1 bit for each modified allocationunit in the disk’s metadata, thus providing an efficient and scalable architecture.Once the disk is repaired or the temporary problem is resolved (that is, cable issue, controller,HBA, and so on), ASM can resynchronize the tracked extents that were modified during outage.In Oracle Database 11g, the time to recover from a perceived disk failure is directly relative tohow many extents have changed during the outage. ASM can quickly resynchronize the changedextents on the failed disk with its surviving disks. The potential increase in performance for thefast disk resync feature is proportional to the number of changed allocation units. Resyncactivities that take hours in Oracle Database 10g can perceivably be down to minutes in OracleDatabase 11g.