oracle internal :VIEW: X$KCBLDRHIST - Direct Read HISTory

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Article-ID:         <Note:159900.1>
Alias:              VIEW:X$KCBLDRHIST
Circulation:        PUBLISHED (INTERNAL) ***Oracle Confidential - Internal Use Only***
Folder:             server.Internals.General
Topic:              ** X$ Table Definitions
Title:              VIEW: X$KCBLDRHIST - Direct Read HISTory - (9.0)
Document-Type:      REFERENCE
Impact:             LOW
Skill-Level:        NOVICE
Server-Version:     09.00
Updated-Date:       05-OCT-2001 05:27:02
References:
Shared-Refs:
Authors:            MEPRESTO.US
Attachments:        NONE
Content-Type:       TEXT/PLAIN
Products:           5/RDBMS (9.0);

View:   X$KCBLDRHIST
         [K]ernel [C]ache [B]uffer [L]oad management
           [d]irect [r]ead [hist]ory

 Column          Type               Description
 --------        ----               --------
 ADDR            RAW(4|8)           address of this row/entry in the array or SGA
 INDX            NUMBER             index number of this row in the fixed table array
 INST_ID         NUMBER             oracle instance number
 DRTHRUPT        NUMBER             throughput of direct reads
 TOTLSLTS        NUMBER             Total number of slots as seen by CKPT

 INCREASE        NUMBER             Increase bit
      KCBLHV_STAY    0        Do not increase number of slots
      KCBLHV_INCR    1        Increase number of slots
      KCBLHV_SYNC    2        Syncronize mode

 REALSLTS        NUMBER             Total number of slots as seen by the PQ slaves

Notes from source:

   One copy of the history vector is allocated per instance.
   Each entry corresponds to a different time snapshot.
   Every 3 seconds, the next entry is updated.
   Each entry contains the following information:
   - Throughput of direct reads during the last 3 seconds expressed in
     blocks per second.
   - Total number of slots (1). The background process every 3 seconds adds
     up the number of slots in all active sessions. This is possible
     since every session maintains statistics about its number of slots.
   - Total number of slots (2). Each slave running adaptive direct reads
     adds regularly its number of slots to the history vector.
     The reason for having two different variables representing the
     same logical entity is because either of those two variables
     alone is not accurate enough. For example, if just before the end
     of the 3 second interval, a new slave started then (1) will not be
     accurate. On the other hand, if a slave just missed the chance to
     add its number of slots in the last entry, then (2) is wrong.
     So before any decision is taken, we make sure that (1) is identical
     to (2).
   - Increase bit. This bit indicates the decision taken by the background
     process for the next 3 seconds interval based on the previous history
     entries. Check the note in kcblclc for more information about the
     increase bit.

KCBLHSZ  1000        Size of History Vector in number of rows

 KCBLHSLTRT  25       Minimum percentage increase in number of slots
Minimum percentage increase in throughput expressed in percentage
increase in the number of slots. For example, if we have a 40% increase
in the number of slots, then the minimum increase in
throughput needs to be 40% * 25% = 10%. So this number
correlates the increase in number of slots with the throughput.
The minimum increase in throughput is used by the background process
as will be seen later. The latter compares the current increase in
throughput with the minimum increase to determine whether to increase
the number of slots in the system or not.

 KCBLHTHPTRT 25
Maximum IO Wait to Cumulative time ratio where slave is CPU bound
expressed in percentage. If that IO wait to cumulative time ratio
is greater than this number, then the slave is considered IO bound
and would increase number of slots if background process indicates
to do so. Otherwise, the slave is considered CPU bound and
consequently ignores all decisions taken by the background process.

 KCBLHCPUBDRT  10
NOTE ON PERSISTENT AND NONPERSISTENT STATE FOR ADAPTIVE READS:
The state for adaptive direct reads is composed of a persistent and
nonpersistent state. Persistent state means that kcbl state is
maintained across uses of a load descriptor. In other words, the
state is maintained across kcbldrini calls. The reason for that is
that sometimes kcbldrini is called multiple times for the same table
or index... We don't want to restart the adaptation from scratch
everytime kcbldrini is called. Instead we would like to maintain some
state so that in case kcbldrini is called again on that same object,
no redundant adaptation is needed.