HP-UX System Administrator's Guide: Logical Volume Management HP-UX 11i v3 (B3921-90053 September 2011) Transform used: ESS version 2.2

The larger the unit of unshare, the more will the latency be when data has to be unshared and the
lesser the metadata space needed on disk to track the sharing relationship between snapshots and
the original logical volume. If the application performs large and occasional writes, then it is
recommended that a larger unshare unit is used. If the writes are small and occasional, then a
smaller unshare unit is recommended. If the write pattern is going to be such that they are small
and sequential, then again a large unshare unit is recommended because after the first unshare,
the following consecutive I/Os that fall within the same unshare unit will not incur the overhead of
a unshare operation.
Refer to the LVM Snapshot Logical Volumes white paper for more information about snapshots and
performance.
Increasing Performance Through Disk Striping
Disk striping distributes logically contiguous data blocks (for example, chunks of the same file)
across multiple disks, which speeds I/O throughput for large files when they are read and written
sequentially (but not necessarily when access is random).
The disadvantage of disk striping is that the loss of a single disk can result in damage to many
files because files are purposely spread across two or more disks.
Consider using disk striping on file systems where large files are stored, if those files are normally
read and written sequentially and I/O performance is important.
When you use disk striping, you create a logical volume that spans multiple disks, allowing
successive blocks of data to go to logical extents on different disks. For example, a three-way
striped logical volume has data allocated on three disks, with each disk storing every third block
of data. The size of each of these blocks is called the stripe size of the logical volume. The stripe
size (in K) must be a power of two in the range 4 to 32768 for a Version 1.0 volume group, and
a power of two in the range 4 to 262144 for a Version 2.x volume group.
Disk striping can increase the performance of applications that read and write large, sequentially
accessed files. Data access is performed over the multiple disks simultaneously, resulting in a
decreased amount of required time as compared to the same operation on a single disk. If all of
the striped disks have their own controllers, each can process data simultaneously.
You can use standard commands to manage your striped logical volumes. For example, the
lvcreate, diskinfo, newfs, fsck, and mount commands all work with striped logical
volumes.
The following guidelines, most of which apply to LVM disk usage, apply to striped logical volumes
for performance reasons:
• Best performance results from a striped logical volume that spans similar disks. The more
closely you match the disks by speed, capacity, and interface type, the better the performance
you can expect. When striping across several disks of varying speeds, performance is no
faster than that of the slowest disk.
• If you have more than one interface card or bus to which you can connect disks, distribute
the disks as evenly as possible among them. That is, each interface card or bus must have
roughly the same number of disks attached to it. You can achieve the best I/O performance
when you use more than one bus and interleave the stripes of the logical volume. For example,
if you have two buses with two disks on each bus, order the disks so that disk 1 is on bus 1,
disk 2 is on bus 2, disk 3 is on bus 1, and disk 4 is on bus 2, as shown in Figure 4.
Planning for Performance 31