Is it possible to have different write-block-size on different machines in cluster? (Our cluster was started with 1M block size but now we are thinking of migrating to 128K and we want to know if it is possible without downtime)
Does write-block-size affect on-disk format? (Can I just change it in config and restart?)
To answer you question, yes different write-block-size configurations can stay in an cluster when increasing the size of the write blocks. You can do a rolling restart of the cluster nodes when doing so.
However, note that you cannot reduce the write-block-size configuration without zeroing the drives. So, in theory you can do a rolling restart without downtime but you would need to wait for migrations to be completed before moving on to the next node to avoid data loss (assuming you have replication factor of 2).