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Sync vs. Async I/O
Disk IO is usually async
- Check with V$BACKUP_SYNC_IO
- Want no rows for your channel here
- Want rows in V$BACKUP_ASYNC_IO
Tape IO is synchronous by default
- Enable async with BACKUP_TAPE_IO_SLAVES
- See example, next page
Notes:
A major performance benefit is the ability of most disk devices to support asynchronous I/O. This means that a channel can issue an I/O to write a block to the disk, then, while that operation is taking place, move on to the next operation. You will definitely want asynchronous I/O. To check if you are achieving asynchronous I/O for your disk devices, query the V$BACKUP_SYNC_IO view after running a backup. If it contains data related to the backup operations channels, you are using synchronous I/O. THIS IS AN ISSUE YOU WILL WANT TO ADDRESS. If this is the case, set initialization parameter DBWR_IO_SLAVES to a non-zero value and allocate additional memory to the large pool. This will cause RMAN to spawn four I/O slaves for backup and restore operations, simulating asynchronous I/O. Refer to Chapter 14 of the Oracle9i Recovery Manager User’s Guide for more information on this issue.
Tape devices, on the other hand, are synchronous devices. Thus, if your backing up or restoring from tapes, you will want to enable support for asynchronous I/O with the BACKUP_TAPE_IO_SLAVES=TRUE initialization parameter. See the next page for an example.