AIX > Administrator > Performance

Tuning a Perfect Note

A look at performance tuning and new AIX 5.3 commands.

A look at performance tuning and new AIX 5.3 commands.

Previously, when you ran vmtune it would tell you the settings for what is currently in use (numperm and numclient). These can now be obtained using the vmstat -v command as can be seen below:

vmstat -v
  20.0 minperm percentage
  80.0 maxperm percentage
  38.9 numperm percentage
  37.2 numclient percentage
  80.0 maxclient percentage
  2 pending disk I/Os blocked with no pbuf
  1717 paging space I/Os blocked with no psbuf
  47887 filesystem I/Os blocked with no fsbuf
  0 client filesystem I/Os blocked with no fsbuf
  310 ext pager filesystem I/Os blocked with no fsbuf

As you can see, both numperm and numclient are reported in percentages - these two fields tell you how much memory is being used by JFS, JFS2, GPFS and NFS filesystems. To figure out the JFS number, subtract numclient from numperm.

The last five lines of the vmstat -v report are useful when you're looking for I/O problems. The first line is for disk I/Os that were blocked because there were no pbufs. Pbufs are pinned memory buffers used to hold I/O requests at the logical volume manager layer. Prior to AIX v5.3, this was a systemwide parameter. It's now tuneable on a volume-group basis using the lvmo command. The ioo parameter that controls the default number of pbufs to add when a disk is added to a volume group is pv_min_pbuf, and it defaults to 512. This specifies the minimum number of pbufs per PV that the LVM uses, and it's a global value that applies to all VGs on the system. If you see the pbuf blocked I/Os field above increasing over time, you may want to use the lvmo -a command to find out which volume groups are having problems with pbufs and then slowly increase pbufs for that volume group using the lvmo command. I normally increase the global value to 1,024.

Paging space I/Os blocked with no psbuf refers to the number of paging space I/O requests blocked because no psbuf was available. These are pinned memory buffers used to hold I/O requests at the virtual memory manager layer. If you see these increasing, then you need to either find out why the system is paging or increase the size of the page datasets.

Filesystem I/Os blocked with no fsbufs refers to the number of filesystem I/O requests blocked because no fsbuf was available. Fsbufs are pinned memory buffers used to hold I/O requests in the filesystem layer. If this is constantly increasing, then it may be necessary to use ioo to increase numfsbufs so that more bufstructs are available. The default numfsbufs value is determined by the system and seems to normally default to 196. I regularly increase this to either 1,024 or 2,048.

Client filesystem I/Os blocked with no fsbuf refers to the number of client filesystem I/O requests blocked because no fsbuf was available. Fsbufs are pinned memory buffers used to hold I/O requests in the filesystem layer. This includes NFS, VxFS (Veritas) and GPFS filesystems.

Finally, ext pager filesystem I/Os blocked with no fsbuf refers to the number of external pager client filesystem I/O requests blocked because no fsbuf was available. JFS2 is an external pager client filesystem. If I see this growing, I typically set j2_nBufferPerPagerDevice=1024.

Jaqui Lynch is an independent consultant, focusing on enterprise architecture, performance and delivery on Power Systems with AIX and Linux.


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