FAQ
I wanted to find out what the average multiblock I/O size. Based on

bytes = "physical read bytes"
total data blocks read = bytes / database block size
single block reads = "physical read IO requests" - "physical read total
multi block requests"
blocks read via multi-block reads = total data blocks read - single block
reads

It seems like

average multi block read size =blocks read via multi-block reads / multi
block read requests

( "physical read bytes"/ (db block size))- ("physical read
total IO requests" -
" physical read total multi block requests"))/ " physical read total multi
block requests"

but wondering if this is right and it seems a little convoluted.

My end goal is to match up I/O latencies off disk with I/O wait events.
Sometimes "db file sequential reads" are slow, sometimes it's the "db file
scattered reads" and sometime its "direct path read". If each of these has
different I/O sizes then with dtrace I can match up the I/O sizes and
latencies to the wait event. I'm thinking it's better to just read the I/O
sizes out of Active Session History per I/O wait type. These I/O sizes will
be skewed to the higher sized, but at least should show the max size and
give some idea about size variation.

PS anyone know what kinds of I/O is included in "physical read total bytes"
- seems to include data block reads, control file reads and redo reads

Kyle
http://dboptimizer.com

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  • K Gopalakrishnan at May 28, 2011 at 12:06 am
    Kyle,

    From the reference, physical read total bytes is "Total size in bytes of
    disk reads by all database instance activity including application reads,
    backup and recovery, and other utilities. The difference between this value
    and "physical read bytes" gives the total read size in bytes by
    non-application workload."

    So I would assume this includes controlfile and redo reads as they are part
    of backup and recovery and other utilities.

    Physical Read Bytes is "Total size in bytes of all disk reads by application
    activity (and not other instance activity) only". I would interpret this as
    pure dbwr and end user SQL IO.

    -Gopal
    PS anyone know what kinds of I/O is included in "physical read total
    bytes" - seems to include data block reads, control file reads and redo
    reads


    - Kyle
    http://dboptimizer.com
    --
    http://www.freelists.org/webpage/oracle-l

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postedMay 27, '11 at 10:32p
activeMay 28, '11 at 12:06a
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