RE: "db file sequential read" latency during reads versus update activity

From: Hameed, Amir <Amir.Hameed_at_xerox.com>
Date: Mon, 12 Mar 2018 19:10:11 +0000
Message-ID: <DM5PR1101MB234752C0D274B9E5B388C3A6F4D30_at_DM5PR1101MB2347.namprd11.prod.outlook.com>

Jonathan,
The IO profile section is shown below from the AWR report:

IO Profile                  Read+Write/Second     Read/Second    Write/Second
~~~~~~~~~~                  ----------------- --------------- ---------------
            Total Requests:          26,686.2        22,378.0         4,308.2
         Database Requests:          26,079.6        21,907.7         4,172.0
        Optimized Requests:               0.0             0.0             0.0
             Redo Requests:             134.5             0.0           134.5
                Total (MB):             232.2           191.4            40.8
             Database (MB):             218.0           184.0            33.9
      Optimized Total (MB):               0.0             0.0             0.0
                 Redo (MB):               6.8             0.0             6.8
         Database (blocks):          27,901.2        23,557.0         4,344.2
 Via Buffer Cache (blocks):          27,890.2        23,549.3         4,340.9
           Direct (blocks):              11.0             7.7             3.4

Thanks
-----Original Message-----

From: oracle-l-bounce_at_freelists.org [mailto:oracle-l-bounce_at_freelists.org] On Behalf Of Jonathan Lewis Sent: Sunday, March 11, 2018 7:03 AM
To: Oracle-L Freelists <oracle-l_at_freelists.org> Subject: Re: "db file sequential read" latency during reads versus update activity

If you're doing updates then you will be generating redo which has to be written, and modifying datablocks and undo blocks which eventually have to be written. How does the number and duration of write operations compare with the number of read operations - it's perfectly feasible that you're reads are slower because they're joining a queue with writes.

Regards
Jonathan Lewis

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From: oracle-l-bounce_at_freelists.org<mailto:oracle-l-bounce_at_freelists.org> <oracle-l-bounce_at_freelists.org<mailto:oracle-l-bounce_at_freelists.org>> on behalf of Hameed, Amir <Amir.Hameed_at_xerox.com<mailto:Amir.Hameed_at_xerox.com>> Sent: 08 March 2018 14:54
To: Oracle-L Freelists
Subject: "db file sequential read" latency during reads versus update activity

I am running SLOB to gauge latency of the storage array. When I configure my test for SELECT only run (no updates), I see average latency of db file sequential read consistently around 3ms. However, as I start to add UPDATE activity to the run, I see average latency of db file sequential read go up in the vicinity of 5ms. Is the cause of this increase in latency due to the commit cleanouts phenomena? The database version is 12.1.0.2. My DB_CACHE size is set to 128M to drive physical IOs and I am running my tests with 64 concurrent users.

Thanks,
Amir

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http://www.freelists.org/webpage/oracle-l Received on Mon Mar 12 2018 - 20:10:11 CET