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

HP-UX Handbook – Rev 13.00 Page 46 (of 110)
Chapter 13 LVM
October 29, 2013
more appropriate when working with disk arrays with large cache sizes, such as the HP
Storageworks XP and EMC models. This is because there is a large difference between
the typical "stripe depth" used with Block striping (often 64Kb or less - though the size
can be as large as 32 MB if desired) and Extent-based striping (from 1 MB to 256 MB);
where intensive sequential I/O takes place, the cache-management algorithms are
likely to work better with the larger stripe depth. With small stripe depths there is a
danger that multiple streams of parallel sequential activity may start to be treated by the
array management algorithms as if they were random access.
In general, it is better to use the term "Extent distribution" when discussing extend
based "striping" - and the objective when setting up LVs in this way is to distribute the
I/O load across multiple storage components in a large storage array with significant
caching of sequential I/O. This is not the same as the approach to striping which was
popular with JBOD disks, where the objective is also to speed up sequential activity,
and where a small stripe "depth" may be appropriate.
Striping and Distributed Allocation Policy
LVM supports two types of striping. The first is block based striping which is configured
using
lvcreate -I <stripe_size> -i <# of stripes>
NOTE: Block based striping is not supported in conjunction with LVM mirroring using
MirrorDisk/UX.
The second striping option is extent distribution. Extent distribution is supported with
LVM mirroring. The logical volumes are set up so that the next logical extent added is
always placed to a different physical volume from the preceding logical extent. 1 MB is
the smallest physical extent size - and it is important to note that the maximum LV size
is determined by the extent size, which cannot be changed after a VG has been created
- there can be a maximum of 64K extents in a single LV.
It is worth noting that the Distributed attribute may be changed with the lvchange(1M)
command, and this includes an option to "force" the attribute onto the LV if its extents
are not already distributed. This could be useful if there is a desire for future extension
of the LV to use extent distribution, or perhaps if there is a need to create an additional
mirror copy with its extents distributed across PVs in another PVG.
Procedure for creation of LVs with extent distribution:
1) Create the VG
# mkdir /dev/vgDist