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Posted Dec 11, 2003

DB2 Instance and Fast Communication Manager - Page 2

By Marin Komadina

DBM Instance Memory Structure

DBM Instance Memory (usually called Global Control Block) has the following internal structure:

Database Manager Instance memory elements:

  • Monitor Heap Size
  • Directory Cache
  • Audit Buffer
  • FCM Memory Allocations
  • Backup and Restore Buffer

Every DB2 instance has the same shown structure. Backup and restore buffers are allocated and deallocated on demand and are not permanently present. The ESE DB2 database with enabled intra_partition or inter-partition parallelism (logical or physical partitioned DB2 database) has allocated an additional memory segment for the Fast Communication Manager (FCM). FCM provides communication support between database partitions.

The database's instance memory allocation is controlled with the following parameters:

Database System Monitor Heap Size


Configurable Online





The Monitor Heap Size is the memory segment used for collecting monitor data. The Monitor heap size is measured in 4KB pages.

# db2 get dbm cfg | grep MON_HEAP_SZ
Database monitor heap size (4KB) (MON_HEAP_SZ) = 56 

Activities such as snapshots, activating or resetting monitor switches and event monitors will populate the memory buffer allocated on the instance startup. Having the DBM setting mon_heap_size=0 will prevent any collection of monitored data. For a heavily used database with many applications connected and a high demand for collecting monitor information, may cause a monitor buffer overflow. Such situations with an inadequate monitoring buffer will result in an error condition, entries in the database alert log and failures on starting additional monitor sessions. The default monitor heap size is 56 in 4KB pages or 224KB. The maximum size for the mon_heap_size is 240 MB or 60000 in 4KB pages.

Directory cache support or database directory listings


Configurable Online





The DBM configuration parameter, dir_cache enables database directory caching. Database directory is the common name for the system db directory, local database directory, node directory and DCS directory. Each of them is used for storing determined content:

  • system database directory contains information related to all known databases
  • local database directory contains information related to all local databases
  • node directory contains connectivity information related to the remote databases
  • DCE directory contains connectivity information for the host databases

Database directory information caching reduces connection delay and directory file I/O.

DBM will automatically govern the shared directory cache, while applications have to manage a separate, private directory cache. The shared cache is populated during DBM (instance) startup.

# db2 get dbm cfg | grep DIR_CACHE
Directory cache support (DIR_CACHE) = YES

After startup, all local and remote database and node entries are cached in the shared cache. Any newly created entry that results from catalog operations (database catalog, uncatalog, create or drop) would not be cached in the shared directory cache. Only on the next DBM restart will new entries be cached. By default, dir_cache parameter is always enabled.

Listing node directory entries for the test DB2 instance:

db2 => list node directory  

Node Directory
 Number of entries in the directory = 2
Node 1 entry:
 Node name                      = ARTIST01
 Comment                        = Article DB2 Server
 Protocol                       = TCPIP
 Hostname                       = ARTIST01.zg.tel.hr
 Service name                   = 50000
Node 2 entry:
 Node name                      = HOST99
 Comment                        =
 Protocol                       = TCPIP
 Hostname                       = IBMHOST01.zg.tel.hr
 Service name                   = 51100

Listing system database directory entries for the test DB2 instance:

db2 => list database directory 

 System Database Directory

 Number of entries in the directory = 3

Database 1 entry:

 Database alias                  = ARTIST
 Database name                   = ARTIST
 Node name                       = JUPITER0001
 Local database directory        = /export/users/db2inst1/node0/artist
 Database release level          = 9.00
 Comment                         = ARTICLE DB2
 Directory entry type            = Indirect
 Authentication                  = SERVER
 Catalog node number             = 0

Database 2 entry:

 Database alias                  = HOST01
 Database name                   = HOST01
 Node name                       = SATURN00010
 Database release level          = 9.00
 Comment                         = IBM Host DB2
 Directory entry type            = Remote
 Authentication                  = DCS_ENCRYPT
 Catalog node number             = -1

Database 3 entry:
 Database alias                  = MARIN
 Database name                   = MARIN
 Local database directory        = /export/users/db2inst1/node0/artist
 Database release level          = 9.00
 Comment                         = ALIAS FOR LOCAL DB 
 Directory entry type            = Indirect
 Authentication                  = SERVER
 Catalog node number             = 0

Remark following specific entries in the system database directory:

1- ARIST (Directory entry type=Indirect) local database entry

2- HOST01 (Directory entry type=Remote) remote Host DB2 database entry

3- MARIN (Directory entry type=Indirect) local database alias

Audit Buffer Size


Configurable Online





The Audit Buffer Size parameter specifies the size for the audit buffer, created during DBM start in the instance control block and used for audit information caching.

# db2 get dbm cfg | grep AUDIT_BUF_SZ
Audit buffer size (4KB)                  (AUDIT_BUF_SZ) = 0

The default value for audit_buf_sz parameter is 0. When this parameter is set to the default of zero (0), DBM will not cache the audit records but will write them ddirectly, synchronously to disk. The Audit buffer values range from a minimum of 0 to a maximum of 65000 in 4KB pages, resulting in a maximum audit buffer size of 254MB.

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