Choosing the Right Disk Technology in a High Availability Environment DRAFT Version 2.0, August 1996

Performance News Notes, Volume 13, No. 1, January/March 1995

DRAFT -- Revision 2.0

August 22, 1996Page 35

whether a mechanism has failed in a RAID 3 or 5 configuration

disk link (HP-FL, SCSI, F/W SCSI)

the number of targets on a disk link

LVM versus non-LVM managed disks

Organizations now usually employ Logical Volume Manager (LVM) to configure disks.

All new high availability products including MC/ServiceGuard and MC/LockManager

require

the disks to be configured with LVM since these HA products use LVM to

enforce exclusive or shared mode activation, and to perform disk locking. LVM typically

consumes 1 - 3 % additional software overhead due to LVM code paths, but this is a

consideration only if the system is 100 % CPU bound. Therefore, although LVM

typically adds only a slight amount of CPU overhead, performance testing should be

done with LVM-configured disks to duplicate the exact customer environment.

Standalone disks work well in almost any I/O environment. There is not really a

suboptimum environment for standalone disks. In contrast, RAID disks excel in certain

environments but provide extremely poor performance in other environments.

Standalone disks usually provide the best performance in all environments.

Striping

Striping of data across multiple disk mechanisms is a standard feature in RAID arrays

and can be performed with standalone disks via LVM. Striping can greatly improve the

speed of random access.

For sequential access, the effect of striping depends upon I/O size. The performance

of small, independent sequential I/Os is unaffected by striping. However, large

sequential I/Os (>= 64 KB) can benefit greatly from striping. HP-UX version 10.0 has

an internal feature called I/O merging that will automatically merge multiple sequential

I/Os that are currently in the disk queue into a single I/O request. This feature

reduces overhead and improves performance by reducing the commands and setup

associated with many I/O requests. Since I/O merging increases the size of the I/O,

performance can be further increased by striping.

Unlike HP-UX versions before 10.0, HP-UX version 10.0 explicitly supports software

striping via LVM with an easy-to-use option to the

lvcreate

and

lvextend

commands,

and can be used to stripe in units as small as 4 KB. In contrast, hardware striping in

RAID arrays is done in units of sectors, which are 512 bytes in size. LVM striping

appears to provide better performance than does hardware striping!