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

  • From: Tim Gorman <tim.evdbt@xxxxxxxxx>
  • To: oracle-l@xxxxxxxxxxxxx
  • Date: Sun, 11 Mar 2018 10:10:07 -0600

>> 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.


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.



Mladen Gogala
Database Consultant
Tel: (347) 321-1217

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