i5/OS storage management

This brief overview of i5/OS™ storage management concepts is intended for administrators who are more familiar with how Windows servers manage storage. Because i5/OS handles storage management differently than a PC server, some techniques that you need in the PC server world are unnecessary in the Windows environment on iSeries™.

i5/OS and disk drives

i5/OS, the operating system that runs on an iSeries, does not need to deal directly with disk drives. Beneath the operating system a level of software (called System Licensed Internal Code (SLIC)) "hides" the disk drives and manages the storage of objects on those disk drives. A virtual address space is mapped over the existing disk space and used for addressing objects rather than disk drive IDs, cylinders, and sectors. Needed objects are copied ("paged in") from this address space on disk into the address space of main memory.

Because of the way i5/OS manages disk data, you do not generally need to worry about partitioning high-growth databases, defragmenting disks, or disk striping on your integrated server. The integrated server uses device drivers to share the i5/OS disk drives. These device drivers send and receive disk data to the i5/OS storage management subsystem. i5/OS storage management handles the hard disks, including spreading the Windows disk drive images across multiple hard disk drives and applying RAID and file mirroring (if configured). Disk defragmentation software manages logical file fragmentation of the hard disk images. Because i5/OS storage management handles these tasks, running a defragmentation program on the integrated server helps primarily in cases where in cases where "critical file system structures" can be defragmented.

Disk pools (ASPs)

In i5/OS physical hard disk drives are pooled together into one storage space called a disk pool, also called an auxiliary storage pool (ASP). If your file system runs out of space, you can add a new hard disk drive to the disk pool, and the new storage space will be available immediately. Every system has at least one disk pool, the system disk pool. The system disk pool is always ASP 1. You can configure additional user disk pools, numbered 2 - 255. You can use disk pools to distribute your i5/OS data over different groups of disks. You can also use this concept to move less important applications or data to your older, slower disk drives. Support for independent ASPs (33-255) is provided through iSeries Navigator. Both the Information Center and iSeries Navigator refer to ASPs as Disk Pools.

Disk protection:

i5/OS disks can be protected in two ways:

• Cross-site mirroring
  Cross-site mirroring, using the operating system geographic mirroring function for IASPs, mirrors data on disks at sites that can be separated by a significant distance.
• RAID-5
  The RAID-5 technique groups several disks together to form an array. Each disk holds checksum information of the other disks in the same array. If a disk fails, the RAID-5 disk controller can re-create the data of the failing disk with the help of the checksum information about the other disks. When you replace a failing disk with a new one, i5/OS can rebuild the information from the failed disk on the new (and therefore empty) disk.
• Mirroring
  Mirroring keeps two copies of data on two different disks. i5/OS performs write operations on both disks at the same time, and can simultaneously perform two different read operations on the two disks of a mirrored pair. If one disk fails, i5/OS uses information from the second disk. When you replace the failing disk, i5/OS copies the data from the intact disk to the new disk.

To further increase the level of protection, you can attach the mirrored disks to two different disk controllers. Then if one controller fails, and with it one set of disks, the other controller can keep the system up. On larger models of iSeries, you can attach controllers to more than one bus. Attaching the two disk controllers that form a mirrored pair to two different buses increases availability even more.

You can define disk pools on i5/OS to have different levels of protection or no protection at all. Then you can put applications and data into a disk pool with the right amount of protection, depending on how important their availability is. For more information about i5/OS disk protection and availability options, read Backup and Recovery.