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

From: Tim Gorman <>
Date: Sun, 11 Mar 2018 10:10:07 -0600
Message-ID: <>

 >> The "db file sequential read" is an index read.

The event "db file sequential read" indicates a single-block random-access synchronous read operation on a datafile.  Even during indexed scans, db file sequential reads alternate between index blocks, table blocks, and undo blocks, and during full table-scans they can start off the operation by reading segment headers, for example.

On 3/10/18 20:01, Mladen Gogala wrote:
> Well, when you have updates, your index block may need to be rebuilt.
> The "db file sequential read" is an index read. If the block has been
> updated, not only does the block need to be rebuilt, the index block
> also needs to be rebuilt. It takes time, even if both are cached in
> memory.  However, the precise answer to your question is not possible.
> For the precise answer, one would need to profile the running code and
> see exactly where the time is spent. Only Oracle developers can do
> that. You may try with strace, to see whether there is another IO that
> somehow creeps in.
> Regards
> On 03/08/2018 09:54 AM, Hameed, Amir wrote:
>> 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 My DB_CACHE size is set
>> to 128M to drive physical IOs and I am running my tests with 64
>> concurrent users.
>> Thanks,
>> Amir
> --
> Mladen Gogala
> Database Consultant
> Tel: (347) 321-1217

Received on Sun Mar 11 2018 - 17:10:07 CET

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