I am running Linux with a harddisk. Currently the blockchain is on an NTFS partition, which probably isn't the fastest. Which filesystem should I choose? As I understand it, a Bitcoin node will only ever append to the blockchain, which means I should not choose a filesystem that is optimized for e.g. fast deletion.
Bitcoin-Core does serval read and write operations. Blocks can also get deleted in case you have enabled
-prune. There are many read operations because your node will very likely serve/relay blocks to other nodes.
The performance of bitcoin-core is more likely constraint by your CPU because of heavy EC verify operations.
Linux does not support NTFS very well, any other Linux native file system will be better.
If you are using NTFS with the FUSE driver it will be very slow.
The most common Linux file system is ext4, I'd just use that :)