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Everything about Table Scans including offload to Exadata, Big Data SQL, and External Tables

  • February 2, 2021

Triaging Smart Scan

Roger MacNicol
Software Architect

This document is my attempt to bring together the available options that can be used to determine the root cause of an issue in order to create a roadmap to help support engineers narrow down the cause of concern.

It is a living document and will be edited and amended as time goes by. Please do check back again in the future.

Warning: these parameters should only be used in conjunction with an Oracle Support Engineer and are not intended for DBAs to self-triage; also they should not be left set after triage without discussion with an Oracle Support Engineer.

The Basics

  1. Check if the issue reproduces without SmartScan
    alter session set cell_offload_processing=FALSE;

    or

    select /*+ opt_param(‘cell_offload_processing’,’false') */ <col> from <tab>;

    This completely turns off Smart Scan, RDBMS will act like non-Exadata and do its own disk I/O through the buffer cache or Direct Read
     
  2. Check if the issue reproduces in cell pass thru mode
    alter session set "_kcfis_cell_passthru_enabled"=TRUE;

    This still uses SmartScan but turns off the smarts, the blocks are read by the offload server and returned unprocessed
     
  3. Check if it reproduces in emulation mode
    alter session set "_rdbms_internal_fplib_enabled"=TRUE;
    alter session set "_serial_direct_read"=TRUE;

    This mode runs the copy of Smart Scan linked into RDBMS to see if the issue stems from offload but not the SmartScan part of it. Be aware: bug fixes and patches are delivered to RDBMS and to the offload server independently - one may have fixes that the other does not and vice versa.
     
  4. There is NO point in trying
    -- alter session set "_kcfis_rdbms_blockio_enabled"=TRUE;

    It simply forces the 'file intelligent storage' layer to divert to the 'direct file' layer i.e. regular block I/O - this achieves exactly the same thing as 'cell_offload_processing=FALSE' but in a round about way.

Triaging issues with Storage Index

  1. Check if the issue reproduces with storage index disabled
    alter session set "_kcfis_storageidx_disabled"=TRUE;

    This will completely disable Storage Index and all chunks will be processed by SmartScan without SI filtering happening.
     
  2. Check if the issue reproduces in Diagnostic mode
    alter session set "_ kcfis_storageidx_diag_mode "=1;

    This will run the query both with and without SI and then compare to make sure SI would have returned the same result for that chunk.
     
  3. Check whether SI Min/Max processing is the issue:
    alter cell offloadgroupEvents = "immediate cellsrv.cellsrv_setparam('_cell_pred_enable_fp_preprocess', 'FALSE')";
     
  4. Check whether any Set Memerbship metadata stored in SI is the issue (this only works in conjunction with the IM format columnar cache a.k.a CC2).
    alter session set "_kcfis_storageidx_set_membership_disabled"=FALSE;

Triaging issues with Flash Cache

  1. If the object is to bypass FC for a single table, the correct way to eliminate FC as a cause is to disable caching for that segment and cause any cached blocks to be flushed.
    alter table <foo> storage( cell_flash_cache  NONE);

    In order to resume default behaviour one would use:
    alter table <foo> storage(cell_flash_cache  DEFAULT );
     
  2. If the goal is to completely bypass the FC layer we need to change the caching policy of the griddisk which will flush the current contents and prevent both write-thru and write-back caching.

    cellcli>ALTER GRIDDISK grid_disk_name CACHINGPOLICY="none";

    In order to resume normal caching policy, one would use:
    cellcli>ALTER GRIDDISK grid_disk_name CACHINGPOLICY="default";
     
  3. Note: the parameter "_kcfis_kept_in_cellfc_enabled" is NOT the correct way to bypass FC because in many cases the disk I/O must go through FC anyway.

Triaging issues with the Columnar Cache

  1. Check if the columnar cache is the cause of the issue:
    alter session set "_enable_columnar_cache"=0;

    and to turn it back on again with default behaviour:
    alter session set "_enable_columnar_cache"=1;

    Note: do not use "_kcfis_cellcache_disabled" - that is not the correct way to triage this.
     
  2. Check if the IM format (a.k.a. CC2) columnar cache is the cause of the issue by forcing version 1 format to be used:
    alter session set "_enable_columnar_cache"=33;  -- 0x01 + 0x20
     
  3. Check whether using the columnar cache with row-major blocks is the cause of the issue
    alter session set "_enable_columnar_cache"=16385; -- 0x01 + 0x4000
     
  4. Check whether using the columnar cache with Hybrid Columnar blocks is the cause of the issue
    alter session set "_enable_columnar_cache"=32769; -- 0x01 + 0x8000

Tracing offload processing

Please see:

  1. Using trace events in an offload server
  2. Tracing Hybrid Columnar Compression Offload
  3. Tracing an Offload Group

Triaging PCODE processing

  1. PCODE is our new byte code for evaluating predicates and aggregates - to go back to the old way use:
    alter session set "_kdz_pcode_flags" = 1;

    and to turn it back on again:
    alter session set "_kdz_pcode_flags" = 0;

Useful Debug Scan values

  1. Disable LOB predicate pushdown to Smart Scan:
    alter session set "_dbg_scan"=1;
     
  2. Disable rowset function evaluation in Smart Scan:
    alter session set "_dbg_scan"=4096;
     
  3. Disable aggregation pushdown to Smart Scan:
    alter session set "_dbg_scan"=8192;
     
  4. Disable Hybrid IM scan - this is where In-Memory is interleaved with Smart Scan
    alter session set “_dbg_scan”=131072;

 

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