HP-UX HB v13.00 Ch-13 - LVM

HP-UX Handbook – Rev 13.00 Page 49 (of 110)
Chapter 13 LVM
October 29, 2013
0998 /dev/dsk/c0t0d0 0499 current /dev/dsk/c1t2d0 0499 current
0999 /dev/dsk/c0t1d0 0499 current /dev/dsk/c1t3d0 0499 current
Notice how PV1 alternates physical extents between the disk devices in PVG1 and
PV2 alternates between the disk devices in PVG2. This is extent based striping.
Comments on Physical Volume Groups (PVGs)
Before the "-D" option of the lvcreate command was introduced, PVGs were generally
only set up for VGs with which Mirrordisk/UX was being used.
PVGs are subdivisions of a Volume Group - they are defined in an ASCII text file,
/etc/lvmpvg, which has a simple format and can be edited manually. The file can be set
up, and amended, by means of the vgcreate, vgextend and vgreduce commands, but it
is also possible to set it up manually with "vi". It is important to note that the names of
PVGs are only held in this file (so they can be changed easily, for instance). When a VG
is exported (using vgexport) its entries are removed from the local /etc/lvmpvg, but they
are not recreated when the VG is imported into the same or another system (using
vgimport)- so manual maintenance is essential in this case, Note that LVs will retain
their "distributed" status, even though no PVG information is imported for the VG by
vgimport.
The default characteristic of a Logical Volume is "strict" allocation policy - this means
that when the volume is mirrored with Mirrordisk/UX, no extent will have its mirror copy
on the SAME PV (this would be irrational, assuming that the mirror copy is being used
for data protection, as is usually the case).
If a Logical Volume is configured as "Group Strict" (by means of the "-s g" option of the
lvcreate command), then mirrored extents must also be in separate PVGs. The typical
approach is to group disks that are connected to the same I/O channel together into
PVGs, so that mirrored extents will not be on the same interfaces - thus interface card
failure will not completely disable access to a mirrored LV. Note that an existing LV can
be made "Group Strict" using the lvchange command, but only BEFORE it is mirrored
(even if it is mirrored and complies with the Group Strict requirement already). In
contrast, it is possible to change a Logical Volume to make it Distributed, or to cease to
be regarded as Distributed.
VGs which are built on disks in arrays, which are independently protected in Mode 1 or
Mode 5, do not normally use Mirrordisk facilities, so they used not to be subdivided into
PVGs. Mirrordisk is not needed to deal with the possibility of interface (I/O channel)
failure with disk arrays, because "Alternate Links" (sometimes known as "PV links") can
usually be set up, and these provide a means for LVM to switch from a primary link to a