System information

328 Logical Partitions on System i5
Logical memory block size
Using this option, you can display or change the Logical Memory Block (LMB) size used by
your system. To display or change the memory block size currently used, you would select
the Logical Memory Block Size option from the Performance Setup menu. You will be
presented with a panel similar to the one in Figure 10-78.
Figure 10-78 Logical memory block size selection
The designation of the logical memory block size you intend to use is a system wide
parameter, so that everything on a given system will be using the same size memory blocks.
In the past we have been able to assign or move memory from a partition at the single MB
level. We could move as little as 1 MB into or out of a partition. Memory on the System i5
machines cannot be manipulated that finely. Instead it must be assigned or moved in blocks.
The LMBs are also sometimes referred to, more accurately, as Memory Regions. This is
similar to the method which the current Model 690 pSeries servers handle memory.
Care should be taken when manually specifying a memory block size, however, because
memory block size selection has an effect on:
򐂰 Initial Program Load (IPL) times
򐂰 Memory granularity
For this reason we recommend that you allow the system to remain at the default of
Automatic.
How logical memory block size affects IPL times
There is some increased IPL times associated with using small LMBs (16MB, or 32MB) when
compared to using the larger sizes of LMBs (64MB, 128MB, or 256MB). So choosing a small
LMB size will increase your IPL time, sometimes dramatically, if this is a very large system.
This is a consideration for all System i5 systems whether the server is being used as a single
server image (a single partition) or as a LPAR machine with several images. If the server was
being used only as a single image you might want to have a larger LMB size that would
produce a shortened IPL time.
How logical memory block size affects memory granularity
We had stated earlier that, in the past, we have been able to assign or move memory from a
partition at the single MB level. We could move as little as 1 MB into or out of a partition.
Memory on the System i5 machines cannot be manipulated that finely. Instead it must be
assigned or moved in blocks. So rather than a memory granularity of 1MB, the System i5
memory granularity will be between 16MB and 256MB. This then becomes the smallest unit
of memory that can be manipulated. This becomes the smallest unit of memory that can be
taken from an LPAR, inserted into an LPAR, or moved between LPARs.