Intel 7xx Servers, 170 Servers, AS/400 RISC Server manual

3.Prefer external disks attached directly to IBM i operating system over those attached via VIOS This is basically a statement of the Fibre Channel adapter and who owns it. In some cases, it affects which adapter is purchased. If you do not need to share a given external disk's resources with non-IBM i operating system partitions, and the support is available, avoiding VIOS altogether will give better performance. First, the disks will usually have 520 byte support. Second, the IBM i operating system support will know the device it is dealing with. Third, VIOS will typically run as a separate partition. If you run VIOS as your first shared partition, simply turning on shared support costs about five to eight percent overall. The alternative, a dedicated partition for VIOS, would be a nice thing to avoid if possible. If you would not have used shared processor support otherwise, or would have to give VIOS a whole processor or more otherwise, this is a consideration.

4.Prefer standard IBM i operating system internal disks to VIOS internal disks. This describes who should own a given set of internal disks. If there is a choice, giving the available internal disks to IBM i operating system instead of going through VIOS will result in noticeably better performance. VIOS is a better fit for external disk products that do not support the IBM i operating system 520 byte sector. The VIOS case would include internal disks that came originally from pSeries or System p. However, one should investigate those devices also. If those devices support 520 byte sectors (or, alternatively, if it is stated they are supported by IBM i operating system), they should be reconfigured instead as native IBM i operating system internal disks. It should be exceptional to use VIOS for internal disks.

5.Prefer RAID 1 or RAID 10 to RAID 5. We are now beginning to generally recommend RAID 1 ("mirroring") or RAID 10 (a "mirroring" variant) for disks generally in On-line Transaction Processing (OLTP) environments. OLTP environments have long had to deal with configurations based on total arm count, not capacity as such. If that applies to you, you have extra space that is of marginal value. Those in this situation can nowadays use the same number of arms deployed as RAID 1 or RAID 10 to gain increased performance. This is at least as true for external disks as it is for internal disks. Note that in this recommendation, one deploys the same arm count -- just deploys them differently, trading unused space for performance. Also note that if one goes this route, two physical disks per RAID 10 or RAID 1 set is better than a larger number of disks per RAID 1 or RAID 10 set. (See also “Ensure, within reason” below).

6.For VIOS, Prefer External Disks (SAN disks) to Internal Disks. SAN disks will have greater flexibility and better tuning options than internal disks. Accordingly, when there is a choice, VIOS is best used for external disks.

7.Separate Journal ASPs from other ASPs. Generally, we have long recommended that a given set of data base files (aka SQL tables) keep its set of journal receivers in a separate ASP from the data base ASP or ASPs. With VIOS, we recommend that this continue to the extent feasible. It may be necessary to share things like Fibre Channel links, but it should be possible to have separate physical devices at the very least. To the extent possible, arrange for journal to use its own internal buses also (of whatever sort the device provides).

IBM i 6.1 Performance Capabilities Reference - January/April/October 2008
© Copyright IBM Corp. 2008	Chapter 14 DASD Performance	218