Trying to plan out some disk usage, mostly media files...

I'll start with four 1TB hard drives (on hand) in RAID6 netting about 2TB. Over time I'm going to replace each of these four 1TB drives with 3TB drives, letting the array rebuild its degraded volume so in the end there will be 2TB + the new 4TB of free space.

The RAID card (Areca) supports growing the VolumeSet to occupy all 6TB but I'm assuming the Linux OS still needs it's partition adjusted somehow to consume and utilize the additional TB's?

Also, are there any glaring obstacles I should know for when bigger drives come out and I grow my storage again down the road?

Now I did ask myself, "do I need one big parking lot" and the answer is "No, it's not mandatory." But what real advantage would I have to carving things up and adding unneeded complexity to the layout? One disadvantage I saw so far was, I'm not keen on having multiple VolumeSets sit in degraded mode while each one rebuilds. They didn't seem to rebuild in parallel when I was tinkering.

The one worry with hardware RAID is that you may not be protected from a card failure as future cards even from the same vendor may not be able to read older RAID disk groups.

If you use LVM it is very flexible about reconfiguring and growing volumes.

The question about rebuild is interesting. The total time to rebuild the whole RAID group should be the same regardless whether the disks are partitioned. The difference is how bad performance is to a volume which is degraded vs one which is being rebuilt. While rebuilding it may have a bitmap of which stripes have been rewritten so once you have accessed one it will be fast. On the other hand, if it has to stop for some reason it is better to not have to rebuild completed RAID sets.

The RAID card I was testing with, I created two Volumes on the disks then pulled a drive and stuck in another. The volumes started rebuilding but did them one at a time. Sure, it may be splitting hairs as the overall repair time is probably the same but I ended up wondering why bothering to carve down the volumes and add that complexity if its not really gaining anything overall.

What file system is recommended on top of LVM for easy file system expansion? And can it be grown "online" vs having to dismount the volume first or use a live cd or something else?

The standard ext3 filesystem can be expanded online.