I'm not sure about the method you're posting about, as it seems like it's mixing between the 2 methods available.
(note: Before you use the 3Tb drive, your BIOS needs to be UEFI or otherwise 3TB capable.)
The simplest method I see for what you have without buying an additional drive, assuming you have a UEFI BIOS would be to setup amahi with your empty 3tb drive with 2 partitions...
1x ?GB for LZ (LZ size is up to you) -- LZ partition -- /var/hda/files
1x remaining GB (remaining size of the drive) -- pooled partition -- /var/hda/files/drives/drive01
This would allow you to offload your smallest drives first and then once they've been offloaded, add them to the pool as well, allowing for the rest of your data to be put in the pool.
After the big transfer is done, you can use gparted from a USB-key and resize the LZ to something smaller if you like.
- do I have to worry about the growing pool-size in terms of long-time side effects
not many people have reported any issues aside from normal hard drive usage degradation.
- am I correct to assume that it is best to add the drives in order of decreasing size?
It doesn't really matter what size the drives are. You can mount them to accommodate your usage choice.
- what will happen if I set the number of copies for a shared folder to a number bigger than my current number of drives? and what will happen if the number is finally reached?
Not entirely sure about this but I imagine greyhole won't let you copy to more than the available drives. When you choose all the available drives, it does copy the files to all available drives.
- why do I need to flush the landing zone manually? the documentation states that this will be done in the background anyway.
Manual LZ -f (flush or file system check) is just the expedited way to do what happens at midnight daily anyways. Doing it manually avoids waiting until midnight before your LZ is free for more files again.
- could I speed this up by using a ram-drive? or would I run out of space without the manual flushes because the daemon does not move the files quickly enough? (that would be inconvenient since this cache partition would necessarily be rather small)
You can only work as fast as your drives can receive and then off-load from the LZ to the pool. I get the feeling a ram-drive would be an un-necessary added step.
- which filesystem should I use?
I use ext4 for all my drives. I have ...
/var/hda/files (1TB, my LZ & my non-pooled storage)
/var/hda/files/drives/drive1 (1TB) - (recently removed & RMA'd due to bad sectors)
/var/hda/files/drives/drive2 (1.5TB)
/var/hda/files/drives/drive3 (2TB)
It's up to you based on what the amahi Wiki says are usable filesystems.
With the scale of storage I have, I see no use for zfs yet as it's just too young.
It's really your own choice depending on the risk you want to take or features you wish to have