About tuning Veritas File System on AIX
|Article:HOWTO65675|||||Created: 2012-01-10|||||Updated: 2012-01-18|||||Article URL http://www.symantec.com/docs/HOWTO65675|
In the absence of D_REFUND mode, the number of PDTs are automatically tuned to 4 times the number of CPUs, up to 64. The total number of VMM buffers are autotuned similarly based on the amount of physical memory of the system. The VMM buffers are then evenly distributed across the PDTs. Hence, if there are either fewer PDTs or a greater number of VMM buffers, then the number of VMM buffers per PDT is larger. The following vxtunefs commands can be used to view the number PDTs and VMM buffers on the system:
# /opt/VRTS/bin/vxtunefs -D print | grep num_pdt # /opt/VRTS/bin/vxtunefs -b
D_REFUND mode faces the limitation of having only 50,000 VMM buffer to allocate dynamically across the PDTs. This can be a limitation in some cases where D_REFUND should be turned off deliberately followed by the appropriate tuning for number of PDTs and VMM buffers.
In the absence of D_REFUND, VxFS performs accounting by allowing the reservation and un-reservation of VMM buffers that are associated with a PDT, before performing an I/O. No such accounting is performed in D_REFUND mode.
Such accounting can sometimes lead to frequent memory allocation, which causes high CPU usage by vx_sched. In such scenarios, you can disable the accounting by setting the vmmbufs_resv_disable tunable using the vxtunefs command.
Article URL http://www.symantec.com/docs/HOWTO65675