HP Process Resource Manager User's Guide
Understanding how PRM manages resources
How PRM manages disk bandwidth resources
Chapter 260
How PRM manages disk bandwidth resources
PRM manages disk bandwidth at the logical volume group/disk group
level. As such, your disks must be mounted and under the control of
either HP’s Logical Volume Manager (LVM) or VERITAS Volume
Manager (VxVM) to take advantage of PRM disk bandwidth
management. PRM controls disk bandwidth by re-ordering the I/O
requests of volume groups and disk groups. This has the effect of
delaying the I/O requests of low-priority processes and accelerating those
of higher-priority processes.
NOTE Disk bandwidth management works only when there is contention for
disk bandwidth, and it works only for actual I/O to the disk. (Commonly,
I/O on HP-UX is staged through the buffer cache to minimize or
eliminate as much disk I/O as possible.) Also, note that you cannot
allocate disk bandwidth shares for PSET PRM groups. PSET PRM
groups are treated as part of PRM_SYS (PRMID 0) for disk bandwidth
purposes.
Disk bandwidth management works on disk devices, stripes, and disk
arrays. It does not work on tape or network devices.
When you change share allocations on a busy disk device, it typically
takes 30 seconds for the actual bandwidth to conform to the new
allocations.
Multiple users accessing raw devices (raw logical volumes) will tend to
spend most of their time seeking. The overall throughput on this group
will tend to be very low. This degradation is not due to PRM’s disk
bandwidth management.
When performing file system accesses, you need approximately six disk
bandwidth consumers in each PRM group before I/O scheduling becomes
noticeable. With two users, you just take turns. With four, you still spend
a lot of your time in system call overhead relative to the peak device
bandwidth. At six, PRM disk bandwidth management begins to take
effect. The more demand you put on the system, the closer the disk
bandwidth manager approaches the specified values for the shares.