To size a SAN Storage Subsystem for SAP means to size for I/O. The needed capacity will be sized afterwards, if necessary at all. Storage performance is the key in all aspects of the sizing process. In any case number of disk is much more important than capacity of disks.

Before we start the sizing we have take into account that all DBs have DATA files and REDO LOG files. Both have to be placed on different disks.
ORACLE writes redo-Logs origlogA and origlogB, as well as the mirror-logs mirrlogA and mirrlogB.

They have to be placed on two RAID10 arrays, each containing only one logical-drive (Disk)
The file system on Disk 1 is holding origlog A and mirrlog B .
The file system on Disk 2 is holding origlog B and mirrlog A .

This combination guarantees best I/O performance.
Usually 2 physical disk are sufficient for one array (RAID1), only larger DB instances need more disks, then configured as RAID10.

The data files (SAPDATA) have to be spread over as many disks as possible, of course cost of these FC Disk have to be taken into account .....

The distribution between read and write access is leading to read, if no further data are available a distribution of 80% read and 20% write can be assumed, with a read hit of 40%. For this IO pattern RAID5 is suitable.

DS4000:
One RAID array should be build of 5 to 10 disks.
At least two RAID-arrays are recommended - this is valid for small system, and needs to be increased dependend on the IO requirements.
It is recommended to put only one logical-drive into one RAID-array, as well as stripping through the LVM

The total Number of disks is depending on the I/O demand:
At best the IO rate should be measured in real live .....
If you have to size for an upgrade use the current measured value, and calculate the increase.

If this is not possible here a rule of thumb: 2 SAPS generates 1 IO with 8 KByte
Through the Tool Disk Magic the number of disk can now be calculated.

For SAP disk with 15k rpm should be used only.

This leads to a absolute min configuration of 4 disks for the redo-logs, and 10 disks for the data file, total 14 disks.
This configuration is suitable just for very small SAP R3 installation, with no more than approx. 2000 SAPS.

This table shows configuration bases on SAPS.
Disk with 15K rpm with a RAID5, each with 5 disks, have been used.