FAQ
I am trying to determine how many physical reads and physical writes
are happening on one of my database servers. I tried using the "sar -
b" command, but the physical reads and writes always appears to be zero.

16:37:26 bread/s lread/s %rcache bwrit/s lwrit/s %wcache pread/s pwrit/s
16:37:31 1 3768 100 6 1508 100 0 0
16:37:36 1 3812 100 4 1546 100 0 0
16:37:41 1 3923 100 2 1589 100 0 0
16:37:46 1 3751 100 7 1522 100 0 0

Environment:
Sun Solaris 9
Oracle 9.2.0.8.0

Any suggestions?

Thanks
Bill

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  • Allen, Brandon at Aug 28, 2009 at 7:54 pm
    Do you have some reason to believe that all your reads aren't really being satisfied from cache?

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  • Bill Zakrzewski at Aug 28, 2009 at 8:03 pm
    This is a 300gb reporting database, very little chance all reads are
    satisfied by the database cache.

    -Bill
    On Aug 28, 2009, at 3:54 PM, Allen, Brandon wrote:

    Do you have some reason to believe that all your reads aren�t really
    being satisfied from cache?


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  • Allen, Brandon at Aug 28, 2009 at 8:13 pm
    Maybe unlikely, but not impossible - just because the database is 300GB doesn't mean that users weren't only querying a small portion of it (e.g. just the most current data) at the time when you were running the sar command. I also work with several DBs in that same size range, but most of it is historical and probably less than 5% of the blocks are used on a regular basis - that's how we get by with just a 3GB buffer cache on a 300GB database. I'm not trying to say that all your blocks are cached, just saying that I wouldn't ignore it as a real possibility so I'd recommend digging deeper to see if it's actually the case or not.

    Regards,
    Brandon

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  • Allen, Brandon at Aug 28, 2009 at 8:15 pm
    I forgot to mention, I'm not talking about the db cache here - those wouldn't show up in the sar output at all, I'm talking about the filesystem buffer cache of your OS (unless you're using direct or raw IO).

    Regards,
    Brandon

    From: Bill Zakrzewski

    very little chance all reads are satisfied by the database cache.

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  • Martin Klier at Aug 29, 2009 at 11:24 am
    Hi,

    do you have perfstat or AWR available?

    If yes, create a report and look up how much gets/reads/db file * read
    your DB and statements do. So you can get an idea what really happens,
    and afterwards compare it with OS metrics.

    Regards
    Martin Klier

    Allen, Brandon schrieb:
    I forgot to mention, I’m not talking about the db cache here – those
    wouldn’t show up in the sar output at all, I’m talking about the
    filesystem buffer cache of your OS (unless you’re using direct or raw IO).



    Regards,

    Brandon



    *From:* Bill Zakrzewski



    very little chance all reads are satisfied by the database cache.




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