Because a single datafile may be striped across 16 disks. In the case of
16 datafiles on 16 disks, Oracle knows that they are separate files, but
it is down to you to set the degree of parallelism, because Oracle can't
know about your underlying physical configuration. With a stripe set, a
single data file is likely to span all disks, therefore all heads would
be used in when Oracle wants to read a lot of blocks sequentially. The
only scope for acceleration on such a configuration would be for the
RAID drivers to make the OS reads underlying the Oracle reads faster,
effectively getting filesystem blocks in parallel (rather than Oracle
blocks) and passing them on.
g
-----Original Message-----
Sent: Thursday, September 20, 2001 3:20 PM
To: Multiple recipients of list ORACLE-L
List,
While reading Gaja's excellent document on Implementing Raid on Oracle
Systems, Gaja mentions the following in his document.
"The degree of striping in the DATA and INDX volumes should also
consider
factors such as data/index partitioning, availability requirements and
support for parallel operations. Please note that 16 datafiles placed on
16
individual drives can support a 16-way parallel operation (if you have
memory and CPU to support it and your controller is not out of capacity
because all 16 drives engaged at the same time). The same cannot be
concluded with 16 datafiles placed on a 16-way RAID volume with 16
drives.
The degree of parallelism that you deploy on a 16-way volume will be
significantly less, when compared to the former configuration of 16
individual drives. This factor should be factored, when making "degree
of
striping" decisions, which in turn controls the number of volumes on
your
system. "
Question:
Why the degree of parallelism is less on a 16-way volume compared to 16
individual drives?
Thanks,
Rao
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