Thursday, April 22, 2010

GPFS Tuning Parameters

GPFS Tuning Parameters is a good wiki information resource written by IBM for GPFS Tuning. Just parroting some of the useful tips I have learned

To view the configuration parameters that has been changed from the default

To view the active value of any of these parameters you can run
mmfsadm dump config

To change any of these parameters use mmchconfig. For example to change the pagepool setting on all nodes.
mmchconfig pagepool=256M

1. Consideration to modify the PagePool

A. Sequential I/O
The default pagepool size may be sufficient for sequential IO workloads, however, a recommended value of 256MB is known to work well in many cases. To change the pagepool size
mmchconfig pagepool=256M [-i]

If the file system blocksize is larger than the default (256K), the pagepool size should be scaled accordingly. For example, if 1M blocksize is used, the default 64M pagepool should be increased by 4 times to 256M. This allows the same number of buffers to be cached.

B. Random I/O
The default pagepool size will likely not be sufficient for Random IO or workloads involving a large number of small files. In some cases allocating 4GB, 8GB or more memory can improve workload performance.
mmchconfig pagepool=4000M

C. Random Direct IO
For database applications that use Direct IO, the pagepool is not used for any user data. It's main purpose in this case is for system metadata and caching the indirect blocks of the database files.

D. NSD Server
Assuming no applications or Filesystem Manager services are running on the NSD servers, the pagepool is only used transiently by the NSD worker threads to gather data from client nodes and write the data to disk. The NSD server does not cache any of the data. Each NSD worker just needs one pagepool buffer per operation, and the buffer can be potentially as large as the largest filesystem blocksize that the disks belong to. With the default NSD configuration, there will be 3 NSD worker threads per LUN (nsdThreadsPerDisk) that the node services. So the amount of memory needed in the pagepool will be 3*#LUNS*maxBlockSize. The target amount of space in the pagepool for NSD workers is controlled by nsdBufSpace which defaults to 30%. So the pagepool should be large enough so that 30% of it has enough buffers.

For more information
  1. GPFS Tuning Parameters
  2. mmchconfig Command

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