LVM Link and Node Failure Recovery Time (June 2009)

LVM Link Failure Recovery Time Formula
The PV links for a PV are all of the lun path device special files (DSFs) configured in LVM for that PV.
At any point in time, one PV link is the active link for the PV. All I/Os are sent to the lun through the
active link lun path. The other PV links are listed as alternate links in the vgdisplay –v command,
and serve as standby links. When an active link fails, LVM selects one of the accessible alternate links
as the new active link. I/Os that failed are retried on the new active link.
The LVM Link Failure Recovery Time is the time LVM takes to detect failed active links and switch all
failed active links to accessible alternate links for all physical volumes (PVs) for all volume groups
(VGs) in the system.
The formula is in seconds.
The formula is valid in both standalone (non-cluster) and Serviceguard cluster configurations.
Note:
The LVM Link Failure Recovery Time is already accounted for in the previously described SLVM
Recovery Time formulas for cases where node failure recovery and link failure recovery are occurring
simultaneously.
Max PV Timeout
The maximum physical volume timeout in seconds for all PVs in all of the VGs on the system.
The smallest Max PV Timeout setting that must be used in the formula is the default PV
timeout value of 30 seconds. Use the pvdisplay command to display the PV timeout setting
for a PV.
This formula assumes the following:
• You have disabled the Multipathing feature of the 11i v3 storage stack by using the
scsimgr command to set the leg_mpath_enable attribute. This enables the LVM PV links
failure recovery feature.
• Legacy lun path DSFs have been configured for the PVs in the case of 11i v3, instead of
persistent lun DSFs.
HP-UX 11i v3 and prior releases
LVM Link Failure Recovery Time = (Max PV timeout * 2) + 15 seconds
This formula is valid on the following HP-UX releases:
• All 11i v3 releases
• All 11i v2 releases with LVM patch PHKL_34094 or superseding patch installed
• All 11i v1 releases with LVM patch PHKL_34093 or superseding patch installed
Example 4
A system has 20 volume groups with 30 PVs in each VG. Each PV has 8 PV links (legacy lun path
DSFs) configured. Access to the active LVM lun path (as displayed by the vgdisplay –v command)
is lost to all 600 PVs. The maximum PV timeout setting is 30 seconds.
LVM Link Failure Recovery Time = (30 seconds * 2) + 15 seconds = 75 seconds
Note:
The minimum value for LVM Link Failure Recovery Time in the formula above is 75 seconds. In
practice, the LVM Link Failure Recovery Time might be less than 75 seconds.
4