VERITAS Volume Manager 3.1 Administrator's Guide

Performance Monitoring

Tuning the Volume Manager

Chapter 9404

which prevents full-stripe read/writes. This throttles the volume I/O

throughput for sequential I/O or larger I/O.

This tunable limits the size of an I/O at the top of the volume manager,

not at the individual disk. For example, If you have an 8x64K stripe,

then the 256K value only allows I/Os that use half the disks in the stripe

and thus it cuts potential throughput in half. If you have more columns

or you’ve used a larger interleave factor, then your relative performance

is worse.

This tunable should be set, as a minimum, to the size of your largest

stripe. This tunable guideline applies to both Raid0 striping and Raid5

striping.

vol_maxkiocount

This tunable controls the maximum number of I/Os that can be

performed by the Volume Manager in parallel. Additional I/Os that

attempt to use a volume device will be queued until the current activity

count drops below this value.

The default value for this tunable is 2048.

Since most process threads can only issue a single I/O at a time, reaching

the limit of active I/Os in the kernel would require 2K I/O requests being

performed in parallel. Raising this limit seems unlikely to provide much

beneﬁt except on the largest of systems.

vol_default_iodelay

This value is the count in clock ticks that utilities will pause for between

issuing I/Os if the utilities have been directed to throttle down the speed

of their issuing I/Os, but have not been given a speciﬁc delay time.

Utilities performing such operations as resynchronizing mirrors or

rebuilding RAID-5 columns will use this value.

The default for this value is 50 ticks.

Increasing this value will result in slower recovery operations and

consequently lower system impact while recoveries are being performed.

voldrl_min_regionsz

With Dirty Region Logging, the Volume Manager logically divides a

volume into a set of consecutive regions. The voldrl_min_regionsz

tunable speciﬁes the minimum number of sectors for a DRL volume