The issue is with the Raid strip. Since your talking 3 drives I assume you’re uing Raid 6 not RAID5 which requires a minimum of 4 disks.
The system strips a portition of some drievs but not necessarily all drives when setting up raid. In raid 5 it uses either four or eight drives for these strips. So if you have 5 140 gb drives in the array the capacity of the first four drives is reduced by 25% to handle the strip. The fifth drive would show the full 140gb. As you add drives to the array the system will not restrip the drives until you add the eight drive. At point all of the capacity of all of the drives will be reduced by 12.5%. If you add more drives (max depends on the controller) all of the drives over eight will always show the full capacity.
RAID 5 is perfectly possible <a href=”http://publib.boulder.ibm.com/infocenter/iseries/v5r4/topic/rzaly/rzalydpp.htm”>with 3 disks under i5/OS</a>, however the main point is still valid. (<a href=”http://publib.boulder.ibm.com/infocenter/iseries/v5r4/index.jsp?topic=/rzaly/rzalyr6c.htm”>RAID 6 requires a 4-drive minimum</a>.)
RAID 5 protects against a single drive failure within each RAID set. It does this by calculating and storing parity data that takes up the equivalent of one drive per RAID set. (The parity data has to be stored somewhere.) A 3-drive set necessarily results in 2-drive capacity.
Parity is stored on a number of drives corresponding to powers of 2, depending on how many drives are in the RAID set — 2 drives in a 3-drive set, 4 drives in 4- through 7-drive sets, 8 drives in 8- through 15-drive sets, and 16 in 16- through 18-drive sets. On a three-drive set, since 2 drives hold parity and since parity occupies 1 drive worth of space, half of two drives is taken — i.e., for 140GB drives, 70GB is taken from 2 of the drives.
Going from three 70GB drives to three 140GB RAID 5 drives gives only 70GB additional capacity — but it also gives RAID 5.