Read errors can be the result of a loss of access to the entire disk or media corruptions on an otherwise a healthy disk. Oracle ASM tries to recover from read errors on corrupted sectors on a disk. When a read error by the database or Oracle ASM triggers the Oracle ASM instance to attempt bad block remapping, Oracle ASM reads a good copy of the extent and copies it to the disk that had the read error.
If the write to the same location succeeds, then the underlying allocation unit (sector) is deemed healthy. This might be because the underlying disk did its own bad block reallocation.
If the write fails, Oracle ASM attempts to write the extent to a new allocation unit on the same disk. If this write succeeds, the original allocation unit is marked as unusable. If the write fails, the disk is taken offline.
One unique benefit on Oracle ASM based mirroring is that the database instance is aware of the mirroring. For many types of logical corruptions such as a bad checksum or incorrect System Change Number (SCN), the database instance proceeds through the mirror side looking for valid content and proceeds without errors. If the process in the database that encountered the read can obtain the appropriate locks to ensure data consistency, it writes the correct data to all mirror sides.
When encountering a write error, a database instance sends the Oracle ASM instance a disk offline message.
If database can successfully complete a write to at least one extent copy and receive acknowledgment of the offline disk from Oracle ASM, the write is considered successful.
If the write to all mirror side fails, database takes the appropriate actions in response to a write error such as taking the tablespace offline.
When the Oracle ASM instance receives a write error message from a database instance or when an Oracle ASM instance encounters a write error itself, the Oracle ASM instance attempts to take the disk offline. Oracle ASM consults the Partner Status Table (PST) to see whether any of the disk's partners are offline. If too many partners are offline, Oracle ASM forces the dismounting of the disk group. Otherwise, Oracle ASM takes the disk offline.
The ASMCMD remap command was introduced to address situations where a range of bad sectors exists on a disk and must be corrected before Oracle ASM or database I/O. For information about the remap command, see "remap".