HP-UX Logical Volume Manager and MirrorDisk/UX Release Notes (762804-001, March 2014)

1.4 Known issues and limitations
This section provides a list of known issues applicable to current and previous releases. It also
includes limitations as known to HP at time of publication. If workarounds are available, they are
included.
1.4.1 Known issues
1.4.1.1 vgmodify fails with I/O error in MMB(Maintenance Mode Boot)
Defect ID QXCR1001327180
Problem In MMB(Maintenance Mode Boot), vgmodify on root VG fails with I/O
error.
Example:
# vgmodify -p 8 -e 10240 /dev/vg00
...
vgmodify: IO error on configuration file
/etc/lvmconf/vg00.conf.temp
Severity Serious
Corrective Action Set VxFS tunable max_direct_iosz to 256 KB as shown below:
vxtunefs -o max_direct_iosz=262144 /
1.4.2 Limitations
This section provides a list of limitations as known to HP at time of publication.
• Logical volumes that have snapshots associated with them might experience an increase in
latencies associated with writes. This is also applicable for writes on (writable) snapshots
themselves.
• With the automatic extent pre-allocation feature available in the HPUX 11iv3 September 2010
update and later releases, when the number of extents remaining in a space-efficient snapshot
logical volume’s pre-allocated pool falls beyond a certain internally computed threshold, by
default, LVM tries to increase the pre-allocated pool size by threshold value. This does not
guarantee that snapshot will not become over-commit. For example, if the I/O rate is faster
than auto pre-allocation, the snapshot becomes over-commit.
If auto pre-allocation is manually disabled, a message is logged in the syslog. This message
is displayed to inform you that further unsharing of data between the snapshot and its successors
might end up depleting the extents in the pre-allocated pool and lead to the snapshot and its
predecessors being marked as inoperative. As soon as you see this message in the syslog,
you must increase the pre-allocated extent pool size for the snapshot logical volume.
• Automatic increase of pre-allocated extents does not prevent snapshots from becoming
inoperative. If the I/O rate is faster than pre-allocation, it is possible that the free extents are
picked before pre-allocation happens and the subsequent I/O marks the snapshot as
inoperative.
• Under very low memory conditions, the removal of a single snapshot using lvremove can
hang if data unsharing is required.
• For more limitations on snapshots, see the Using LVM Logical Volume Snapshots white paper.
1.5 Installation requirements
This section describes the installation requirements for this release.
1.4 Known issues and limitations 9