Re: db file sequential/scattered read - physical or logical io or both?

From: Kellyn Pedersen <>
Date: Mon, 14 Jun 2010 12:46:16 -0700 (PDT)
Message-ID: <>

You should also think about tracing this and find out if the db file sequential read waits are on a dictionary object.  The waits could be a bit misleading on what it is it's actually *waiting* for... :)

Kellyn Pedersen
Sr. Database Administrator
I-Behavior Inc.
"Go away before I replace you with a very small and efficient shell script..."

  • On Mon, 6/14/10, Oracle Dba Wannabe <> wrote:

From: Oracle Dba Wannabe <> Subject: Re: db file sequential/scattered read - physical or logical io or both? To:
Date: Monday, June 14, 2010, 6:36 AM

Hi All,
Thanks for the responses. The report itself is actually a problem - to resolve it I understand I need to address the number of i./o's issued by it. That said, I posed this question because I just wanted to try and understand how oracle issues reads. The reason the naming of the wait events confused me - was I've also understood them to mean physical reads. What didn't make sense to me was why after the report completed and I submitted it again there were still wait events for reads issued in awr - I would have thought the data was cached. I had traced each run of the session, but had just not looked into the trace file yet (my mistake) and was only looking at the awr file. Looking at the trace I see the following:

call     count       cpu    elapsed       disk      query    current        rows
------- ------  -------- ---------- ---------- ---------- ----------  ----------
Parse        1      0.00       0.00          0          0          0           0
Execute      1      0.00       0.00          0          0          0           0
Fetch    90966   2752.59    2690.45          0   43989168          0      727721
------- ------  -------- ---------- ---------- ---------- ----------  ----------
total    90968   2752.59    2690.46          0   43989168          0      727721

Which appears to indicate zero disk reads. However AWR, shows the following:

Avg Wait(ms)
% Total Call Time
Wait Class

db file sequential read
User I/O
There's nothing else running on this system other than this SQL (it was a weekend afterall). So although my trace file shows no physical read events - how do I account for the 1.1 million wait events - The load profile on awr looks like this:

Per Second
Per Transaction

Logical reads:


Block changes:


Physical reads:

So are the db file sequential reads a result of waits from reads at the storage cache? the file system cache (have filemin_cache at 3% and filemax_cache 10% total memory =120GB), the oracle buffer cache or all three? Or is it just the first 2 and does not include the buffer cache requests as a call for data in the buffer cache does not translate to a read wait event? A bit more detail on this system: The server is a 2 Node RAC ( on HP-UX 11.31, Super Dome, the storage is XP 24k, the db cache size is approx 60GB on each node. Thanks once again.

From: Greg Rahn <> To:
Sent: Mon, June 14, 2010 11:23:24 AM
Subject: Re: db file sequential/scattered read - physical or logical io or both?

When you run the report via SQL*Plus with autotrace on, (or look at the session stats) what is the physical reads & consistant gets numbers?  Likely the filesystem cache is providing the "lift" in performance.

On Sun, Jun 13, 2010 at 3:31 PM, Oracle Dba Wannabe <> wrote:
> So I have an idle system, where I run a sql report. The report takes an hour to run. I look at the awr report (30 min interval), and see the io wait events for sequential and scattered reads. The first has an avg wait time of 7ms the second 10ms. These waits as I understand it are physical io requests - correct? The p1 and p2 parameters point to file and block numbers so I guess that makes sense. Anyway I rerun the same report, look at the new awr and now see the same wait events, only with much smaller wait times. Which means data was read from cache - if that's the case why are the same wait events issued? it seems a bit confusing that way.

Greg Rahn

Received on Mon Jun 14 2010 - 14:46:16 CDT

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