order to achieve optimal performance, the placement of database objects should 
be carefully planned. 
...
insert 
update
delete
commit
insert
...
Buffer pool
Log buffer
Data and indexes
Logical log files
Figure 9 6   Explaining logical log
As shown in Figure 9 6, all data modifications are not only written to table space 
containers, but are also logged to ensure recoverability. Because every 
INSERT
, 
UPDATE
, or 
DELETE
 is replicated in the transactional log, the flushing speed of the 
logical log buffer can be crucial for the entire database performance. To 
understand the importance of logical log placement, you should keep in mind that 
the time necessary to write data to disk depends on the physical data distribution 
on disk. The more random reads or writes are performed, the more disk head 
movements are required, and therefore, the slowest is the writing speed. 
Flushing logical log buffer to disk by its nature is sequential and should not be 
interfered by other operations. Locating logical log files on separate devices 
isolates them from other processes, and ensures uninterrupted sequential writes.
To change logical log files to a new location you need to modify the 
NEWLOGPATH
database parameter as shown in Example 9 30. The logs will be relocated to the 
new path on the next database activation (this can take some time to create the 
files).
Example 9 30   Relocation of logical logs
db2 update db cfg for sample using NEWLOGPATH /db2/logs
 Chapter 9. Testing and tuning 
299






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