Veritas Volume Manager 4.1 Administrator's Guide (HP-UX 11i v3, February 2007)

Chapter 15, Performance Monitoring and Tuning
Performance Monitoring
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If two volumes (other than the root volume) on the same disk are busy, move them so that
each is on a different disk.
If one volume is particularly busy (especially if it has unusually large average read or
write times), stripe the volume (or split the volume into multiple pieces, with each piece
on a different disk). If done online, converting a volume to use striping requires sufficient
free space to store an extra copy of the volume. If sufficient free space is not available, a
backup copy can be made instead. To convert a volume, create a striped plex as a mirror of
the volume and then remove the old plex. For example, the following commands stripe
the volume archive across disks mydg02, mydg03, and mydg04 in the disk group, mydg,
and then remove the original plex archive-01:
# vxassist -g mydg mirror archive layout=stripe mydg02 mydg03 \
mydg04
# vxplex -g mydg -o rm dis archive-01
After reorganizing any particularly busy volumes, check the disk statistics. If some
volumes have been reorganized, clear statistics first and then accumulate statistics for a
reasonable period of time.
If some disks appear to be excessively busy (or have particularly long read or write times),
you may want to reconfigure some volumes. If there are two relatively busy volumes on a
disk, move them closer together to reduce seek times on the disk. If there are too many
relatively busy volumes on one disk, move them to a disk that is less busy.
Use I/O tracing (or subdisk statistics) to determine whether volumes have excessive
activity in particular regions of the volume. If the active regions can be identified, split the
subdisks in the volume and move those regions to a less busy disk.
Caution Striping a volume, or splitting a volume across multiple disks, increases the
chance that a disk failure results in failure of that volume. For example, if five
volumes are striped across the same five disks, then failure of any one of the
five disks requires that all five volumes be restored from a backup. If each
volume were on a separate disk, only one volume would need to be restored.
Use mirroring or RAID-5 to reduce the chance that a single disk failure results
in failure of a large number of volumes.
Note that file systems and databases typically shift their use of allocated space over time,
so this position-specific information on a volume is often not useful. Databases are
reasonable candidates for moving to non-busy disks if the space used by a particularly
busy index or table can be identified.
Examining the ratio of reads to writes helps to identify volumes that can be mirrored to
improve their performance. If the read-to-write ratio is high, mirroring can increase
performance as well as reliability. The ratio of reads to writes where mirroring can
improve performance depends greatly on the disks, the disk controller, whether multiple