AIX: I/O Tuning Parameters

Aug 20

I/O Tuning Parameters:

numfsbufs (vmtune –b) specifies the number of file system buffer
structures. This value is critical asVMM will put a process on the wait list
if there are insufficient free buffer structures.

Run vmtune –a (pre 5.2) vmstat –v (5.2 & >) and monitor
fsbufwaitcnt. This is incremented each time an I/O operation has to wait for
file system buffer structures.
A general technique is to double the numfsbufs value (up to a maximum of
512) until fsbufwaitcount no longer increases. This value, as it is dynamic,
should be re-executed on boot prior to any mount all command.
hd_pbuf_cnt (vmtune –B) determines the number of pbufs assigned to LVM.
pbufs are pinned memory buffers used to hold pending I/O requests.
Again, examine vmtune –a and review the psbufwaitcnt. If increasing,
multiply the current hd_pbuf_cnt by 2 until psbufwaitcnt stops incrementing.
Because the hd_pbuf_cnt can only be reduced via a reboot (this is
pinned memory) – be frugal when increasing this value.

I/O Tuning :

Over 35% I/O wait should be investigated.
Oracle databases like async I/O, DB2 & Sybase do not care (a good
place to start would be AIO PARMS of
MINSERVERS = 80 MAXSERVERS = 200 MAXREQUESTS = 8192)
Recent technology disks will support higher ltg numbers
lvmstat (must be enabled prior to usage) provides detailed information for
I/O contention
filemon is an excellent I/O tool (trace – ensure you turn it off)
numfsbufs and hd_pbuf_cnt adjusted to reduce wait counts in vmtune or
vmstat -v