IBM DS8000 manual LVM configuration, AIX access methods for I/O, LVM striping, LVM mirroring

Refer to the manpages of the MPIO commands for more information.

LVM configuration

In AIX, all storage is managed by the AIX Logical Volume Manager (LVM). It virtualizes physical disks to be able to dynamically create, delete, resize, and move logical volumes for application use. To AIX our DS8000 logical volumes appear as physical SCSI disks. There are some considerations to take into account when configuring LVM.

LVM striping

Striping is a technique for spreading the data in a logical volume across several physical disks in such a way that all disks are used in parallel to access data on one logical volume. The primary objective of striping is to increase the performance of a logical volume beyond that of a single physical disk.

In the case of a DS8000, LVM striping can be used to distribute data across more than one array (rank).

Refer to Chapter 12, “Performance considerations” on page 253 for a more detailed discussion of methods to optimize performance.

LVM mirroring

LVM has the capability to mirror logical volumes across several physical disks. This improves availability, because in case a disk fails, there will be another disk with the same data. When creating mirrored copies of logical volumes, make sure that the copies are indeed distributed across separate disks.

With the introduction of SAN technology, LVM mirroring can even provide protection against a site failure. Using long wave Fibre Channel connections, a mirror can be stretched up to a 10 km distance.

Another application for LVM mirroring is online (non-disruptive) data migration. See Chapter 16, “Data migration in the open systems environment” on page 333.

AIX access methods for I/O

AIX provides several modes to access data in a file system. It may be important for performance to choose the right access method.

Synchronous I/O

Synchronous I/O occurs while you wait. An application’s processing cannot continue until the I/O operation is complete. This is a very secure and traditional way to handle data. It ensures consistency at all times, but can be a major performance inhibitor. It also doesn’t allow the operating system to take full advantage of functions of modern storage devices, such as queueing, command reordering, and so on.

Asynchronous I/O

Asynchronous I/O operations run in the background and do not block user applications. This improves performance, because I/O and application processing run simultaneously. Many applications, such as databases and file servers, take advantage of the ability to overlap processing and I/O. They have to take measures to ensure data consistency, though. You can configure, remove, and change asynchronous I/O for each device using the chdev command or SMIT.

352DS8000 Series: Concepts and Architecture