Planning For Performance; General Performance Factors; Memory Usage; Cpu Usage - HP -UX 11i Administrator's Manual

Planning for Performance

This section describes strategies to obtain the best possible performance using LVM. It addresses
the following topics:
"General Performance Factors" (page 29)
"Internal Performance Factors" (page 29)
"Increasing Performance Through Disk Striping" (page 31)
"Increasing Performance Through I/O Channel Separation" (page 33)

General Performance Factors

The following factors affect overall system performance, but not necessarily the performance of
LVM.

Memory Usage

The amount of memory used by LVM is based on the values used at volume group creation time
and on the number of open logical volumes. The largest portion of LVM memory is used for extent
maps. The memory used is proportional to the maximum number of physical volumes multiplied
by the maximum number of physical extents per physical volume for each volume group.
The other factors to be concerned with regarding memory parameters are expected system growth
and number of logical volumes required. You can set the volume group maximum parameters to
exactly what is required on the system today. However, if you want to extend the volume group
by another disk (or perhaps replace one disk with a larger disk), you must use the vgmodify
command.

CPU Usage

Compared to the non-LVM case, no significant impact to system CPU usage (by observing idle
time) has been observed.
With LVM, extra CPU cycles are required to perform mirror write consistency cache operations,
which is the only configurable option that impacts CPU usage.

Disk Space Usage

LVM reserves some disk space on each physical volume for its own metadata. The amount of space
used is proportional to the maximum values used at volume group creation time.

Internal Performance Factors

The following factors directly affect the performance of I/O through LVM.

Scheduling Policy

The scheduling policy is significant only with mirroring. When mirroring, the sequential scheduling
policy requires more time to perform writes proportional to the number of mirrors. For instance, a
logical volume with three copies of data requires three times as long to perform a write using the
sequential scheduling policy, as compared to the parallel policy. Read requests are always directed
to only one device. Under the parallel scheduling policy, LVM directs each read request to the
least busy device. Under the sequential scheduling policy, LVM directs all read requests to the
device shown on the left hand side of an lvdisplay

Mirror Write Consistency Cache

The purpose of the Mirror Write Consistency cache (MWC) is to provide a list of mirrored areas
that might be out of sync. When a volume group is activated, LVM copies all areas with an entry
in the MWC from one of the good copies to all the other copies. This process ensures that the
mirrors are consistent but does not guarantee the quality of the data.
v output.
Planning for Performance 29