To get different batches shouldn't you buy a popular hdd with time between? Like one now and another one in three months from the same (large) retailer? Since so many HDDs of that type has been sold in that time period a new batch is the only likely thing.
Well, yes. But I need to buy at least five drives at a time (minimum) since I use a 3+2 configuration (2 redundant drives per five drives).
And buying all five at once would mean that each slice (which is where batch allocation really matters) would still be the same batch.
One alternative, if you trust the drives to last a few months at least, is to use mirroring instead of raidz2, and buy the mirror drives later (that is, set it up as a normal non-redundant stripe first, then add redundancy a few months later on).
Considering that one out of my 20 drives failed the first week that is probably not a good idea, though.
You could set up a smaller mirrored pool, then add more drives, copy the data to the new drives, verify the copy, split up the old pool, and re-add it as mirrors to the new drives. I guess. Sort of a lot of work, though.