||Oracle Database Tips by Donald Burleson
ASM Disk Load balancing (SAME)
Oracle has been promoting the concept of SAME - Stripe and Mirror Everything, which aimed at using the full I/O
capacity of the all the disks. Many database users have implemented
"SAME sets", each of about 4 mirrored disk pairs. However, the SAME
functionality does not completely eliminate the disk imbalances.
There are different types of files where the IO activity is not
uniform, and the configuration of SAME is not the cure for all IO
ASM goes a step forward, in balancing the IO
activity. Rebalancing distributes file data evenly across all the
disks of the ASM Disk Group. ASM automatically rebalances a disk
group whenever disks are added or dropped. ASM ensures that a file
is evenly spread across all disks in a disk group when the file is
allocated, so rebalancing is not required except when the storage
With the I/O balanced whenever files are
allocated and whenever the storage configuration changes, you never
need to search for hot spots in a disk group and manually move data
to restore a balanced I/O load. It also allows the addition of disks
online. There are extra background processes that conduct the
rebalancing activity in a non-intrusive way.
Thus, the ASM makes manual load balancing
unnecessary, because the dynamic rebalancing overcomes the major
drawback of SAME. When new disks are added to a disk group,
rebalancing happens automatically without an outage.
The above text is
an excerpt from:
Oracle 10g Grid & Real Application
Computing with RAC
by Mike Ault, Madhu Tumma
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