HP-UX Logical Volume Manager and MirrorDisk/UX Release Notes HP-UX 11i v3 (September 2010)

LVM logs a message and tries to increase the pre-allocated extent
pool size by threshold value.
In the case where the stripe size is two (or small), the default threshold
value is reached when there are only 2 pre-allocated extents free.
When this threshold is reached and if auto pre-allocation is enabled,
LVM tries to increase the pre-allocated pool size only by 2 extents,
which is very small. If the I/O rate is faster than the pre-allocation,
it is possible that those two extents are picked before the automatic
pre-allocation happens and the subsequent I/O marks the snapshot
as over-committed and inoperative.
Severity: Medium
Corrective Action: Manually configure the threshold value using lvcreate or
lvchange with the –P option.
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 HP-UX 11i v3 September 2010 Update release, 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.
• For more limitations on snapshots, see the Using LVM Logical Volume Snapshots white paper.
Installation requirements
This section describes the installation requirements for this release.
Required hardware
LVM and MirrorDisk/UX have no hardware requirements beyond the requirements of the HP-UX
11i v3 operating system. Both products run on all supported HP 9000 and HP Integrity servers.
Required software
Because LVM is installed with the HP-UX 11i v3 operating environment, there are no software
requirements beyond the requirements of the HP-UX 11i v3 operating system. MirrorDisk/UX
requires the BaseLVM™ product.
Required patches
In HP-UX 11i v3, LVM and MirrorDisk/UX do not require any software patches.
For each of the following subsystems, you can download patches from the following website:
http://www2.itrc.hp.com/service/patch/mainPage.do
8 Logical Volume Manager and MirrorDisk/UX Release Notes