suppose we have 2 disks of unequal size, once 256G and one 512G. Is there any operation to move data from the 256G disk to the 512G disk? We will eventually remove the 256G disk.
Yes you can do that.. You will just copy your data from your 256gb sized disk to your 512gb sized disk..
"Is there any operation to move data from the 256G disk to the 512G disk?" -> what do you mean? please revise.
What is your disk group configuration? Did you put those disks into the same disk group? If so, I guess you are already using the half of that 512GB sized disk? So I don't see any problem with that.
I had a look at the rebalance operation.. OSASHARE is the diskgroup which needs to be tuned. If IO rebalance is not 100%, then if I remove the 256G, there might be data loss. How do I proceed?
What do you measure with that "IO_BALANCED" value? Probably I/O activity, not the percentage of mirrored extents.. ASM ensures that the extents are equally distributed across all the disks in a diskgroup.
Please follow -> Script to Report the Percentage of Imbalance in all Mounted Diskgroups- Non Exadata/ODA (Doc ID 367445.1)