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Oracle
Metric Average Read Time
Oracle Tips by Burleson Consulting
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The
Average Read Time Oracle metric is the
amount of time spent
for each read against the datafile. This number may be as important
as the number of reads against the file. By comparing read times
across multiple datafiles will show you which datafiles are slower
than others.
Disk read times may be improved if contention
is reduced on the datafile, even though read times may be high due
to the file residing on a slow disk. You should identify whether the
SQL accessing the file can be tuned, as well as the underlying
characteristics of the hardware device.
Mike Ault notes how to use the average read
time statistics
here:
In fact, a good argument can be made for
tying the stripe width to the max_io_size for the platform
(usually a value between 64K and 1 megabyte). We tested a NT4
-6a system against a RAID5 6X1 disk array by matching the value
of db_file_multi_block_read_count times db_block_size against
the stripe width. We achieved a IO time savings of 70%.
The IO timing results showed that the
average read time dropped from 0.279 milliseconds to 0.264
milliseconds per read, the major change being in the average
write time which dropped from an average of 5.646 milliseconds
to 1.324 milliseconds. Putting this in perspective, for a
process which writes out 1,000,000 records this could be a
savings of 1 hour and 12 minutes if they were all single record
writes. Of course, Oracle batches writes so the actual savings
will be less.
You may also
want to consider information provided by
Oracle MetaLink in that you " can find out the average time
taken for I/O from AVGIOTIM in V$FILESTAT"
This, and many other Oracle performance metrics are discussed in
my book "Oracle
Tuning" by Rampant TechPress. You can buy it directly from
the publisher and save 30% at this link:
http://www.rampant-books.com/book_2005_1_awr_proactive_tuning.htm
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