PROMON shows "LRU2 replacement policy disabled" when OS reads GT -B2 value

PROMON > R&D > 2 > 3 shows "LRU2 replacement policy disabled" when OS reads GT -B2 value when the Alternate Buffer Pool is set against database Objects in Type II Storage Areas

Feb 11, 2014
Knowledge

Title

URL Name

000045605

Article Number

000192734

Environment

Product: OpenEdge Version: 10.2B07, 10.2B08, 11.2.x, 11.3.x OS: All Supported Platforms

Question/Problem Description

PROMON > R&D > 2 > 3 Buffer Cache is not showing that the LRU2 replacement policy is enabled after a TABLE-SCAN while also showing that the "Alternate Buffer Pool", "O/S reads" are greater than the current -B2 value.

Querying _tablestat VST confirms that _tablestat-OSread are higher than the value that -B2 is set to.

When a full table read is repeated, PROMON "O/S reads" double.

When the database is started with a larger -B2 value to accommodate the assigned database objects, a full table query run multiple times shows the same value for PROMON "O/S reads".

Steps to Reproduce

Clarifying Information

Alternate Buffer Pool set for database objects (Table, Index, LOBS) through Database Administration.

PROMON > R&D > 4 > 4. Adjust Latch Options, shows that the LRU2 switch is set to "ENABLE" confirming that the LRU2 mechanism is not enabled:
"3. Enable LRU2 alternate buffer pool replacement policy".

Monitoring if the LRU2 chain is enabled cannot be reliably achieved through PROMON.

Private Buffers are not in use (-Bp)
PROBKUP online has not been run.

Error Message

Defect Number

Defect PSC00285575

Enhancement Number

Cause

When the Alternate Buffer pool is assigned at the database OBJECT level (which can only be done for Type II Storage Area objects), then PROMON > R&D > 2 > 3 Buffer Cache correctly reports: "LRU2 replacement policy disabled".

This is not PROMON a display problem. The display is correctly reporting that the lru2 mechanism is not enabled even though it should be when buffers assigned to the Alternate Buffer Pool exceed the current -B2 value.

With the lru2 mechanism not enabled, this creates instead a FIFO replacement policy which may have adverse performance effects as the oldest buffers or "first read in buffers" will be evicted off the LRU2 chain rather than the least recently used buffers. This is observed by the fact that when a full table read that is assigned to the Alternate Buffer Pool (-B2) is repeated, PROMON "O/S reads" double.

Resolution

Upgrade to OpenEdge 11.3.2 or later. Where the LRU2 mechanism is appropriately enabled when the Alternate Buffer Pool is assigned at the database objects level.

Workaround

Workaround 1:

If there is a stated requirement to monitor if the LRU2 mechanism has been enabled in production, then the Alternate Buffer Pool assignments will need to be unassigned at the Database Object level (through Database Administration) and re-assigned at the Storage Area through the command line:
$ proutil dbname -C enableB2 <storage_area_name>.
This may also involve moving the required Alternate Buffer Pool objects to Storage Areas that only contain -B2 assignment objects. By having Objects assigned at the Storage Area level, the LRU2 mechanism will be correctly reported when enabled and the LRU2 mechanism will be properly initialized when -B2 is exceeded

Workaround 2:

Rely on the correct initial Alternate Buffer Pool sizing exercise. Monitoring with _tablestat/_indexstat is only reliable where object READS are concerned. Object writes for example in the Alternate Buffer Pool will reserve -B2 space and may result in the LRU2 being enabled.

Initial sizing of the LRU2 size needs to be undertaken in a controlled fashion.

For Tables, either by loading each "Alternate Buffer Pool" table to a variable length database extent, then dividing the file extent size on disk by the database blocksize, or relying on _tablestat-OSread values after a full table scan.

For Indexes, while _indexstat-OSread could similarly be used, it is easier to simply find the number of index blocks occupied through an IDXANALYS report. The number of blocks for a given index are reported in the "INDEX BLOCK SUMMARY FOR AREA" section under the BLOCKS header

Example:

$ PROUTIL dbname -C idxanalys

There are 7 index blocks for the Customer.Name index.

INDEX BLOCK SUMMARY FOR AREA "Cust_Index" : 10
-------------------------------------------------------
Table Index Fields Levels Blocks Size % Util Factor
PUB.Customer
Name 15 1 2 7 22.5K 80.8 1.4

The same information (in more detail) can be obtained through the IDXBLOCK report:

Example:

$ PROUTIL -C idxblock Customer.Name

There are 7 index blocks for the Customer.Name index.

BlockSize = 4096 Block Capacity = 4004

Number Length On Length Delete

of of Delete of Chain Percent

DBKEY Level Entries Entries Chain Size Type Utilized

44 1 6 68 0 0 root 1

46 2 196 3970 0 0 leaf 99

45 2 183 3982 0 0 leaf 99

112 2 189 4001 0 0 leaf 99

113 2 185 3993 0 0 leaf 99

120 2 232 3998 0 0 leaf 99

121 2 132 2966 0 0 leaf 74

Notes

Keyword Phrase

Last Modified Date

2/11/2014 8:40 AM

PROMON shows "LRU2 replacement policy disabled" when OS reads GT -B2 value

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