I have upgraded the drives in a 4 drive Raidz array in a NAS server and the array has auto expanded to the new size. In testing transfer rates from the NAS server now I observe that when performing large read transfers the performance is poor, struggling to get above 10MB/s although iotop and zpool iostat can report very high numbers up to 500MB/s What could be leading to this strange behaviour? As I have been upgrading the array I can also observe that it performs the same whether there are 4 devices online or degraded to 3.
The server is connected with a single gigabit network connection and can saturate it with a single drive using ext4