SAP Note 24189 - Informix: Disk layout / moving dbspaces / chunks

Symptom:

Bad performance due to unbalanced I/O.

Cause and prerequisites

Unfavorable dbspace/chunk distribution on the hard disks.

Solution
Dbspace/chunk layout

Different criteria can be used as a base for favourable distribution of the Informix dbspaces/chunks on the hard disk.

1. Distribution with regard to I/O rate on dbspaces.

'Early Watch' analyses have shown that the following distribution can be recommended on account of the I/O rates.

• Put ROOTDBS, PHYSDBS and LOGDBS on 3 separate, fast hard disks.

• Put their respective MIRRORS on 3 separate, fast hard disks as well.
  It is mandatory to mirror ROOTDBS, PHYSDBS und LOGDBS.

• PSPABTAB, PSAPCLU and PSAPSTAB have high I/O rates and should therefore be kept separate. Use fast hard disks if possible. They can be combined with dbspaces that are used less often (see below).

• PSAPSOURCE, PSAPDDIC, PSAPDOCU, PSAPLOAD, PSAPESxxx and PSAPELxxx, have low I/O rates and should also be separated in order to avoid unbalanced disk utilization. Slower disks can be used here.
  

2. Heuristic rules

• You should not put dbspaces with high I/O rates on disks which are used as system SWAP. This can result in very poor performance if the disks are used as SWAP area and for dbspaces LOGDBS and PHYSDBS.

• As of Release 3.1G, three temporary dbspaces with 400 MB each are created by R3INST. The temporary dbspaces should be located on different disks, and they should only contain one chunk each. (also see Note 49381).
  

Influence on disk configuration with UNIX-R3INST

The R/3 Standard Installation program (R3INST) can approach optimal disk configuration only if the dbspaces in the file INSTINF.PFL are sorted according to access frequency: for example,
rootdbs \ /
physdbs \ /
logdbs* \ /
tmpdbs* \ /
psapbtab \ /
psapclu \ /
psapstab \ /
psappool \ /
psapprot \ /
psapsource \ /
psapddic \ /
psapdocu \ /
psapload \ /
psapuser1 \/
Optimizing the distribution of dbspaces/chunks MUST be done manually: modify the buildinf.pfl file before the db-build step: ('Shall dbspaces be created?').

Subsequent changes to the disk configuration is currently not supported by Informix.

The only solution is as follows:
Unload all the tables of a dbspace, delete them, create them again and load them. We recommend that you use the DB administration tool SAPDBA to do this.

Additional key words

INF, DBA, installation, PERF, layout, dbspace, chunk