Slow expansion of RAIDZ1

[LarsR]

Maybe this post helps

RAIDZ expansion speedup

I am expanding a RAIDZ2 pool from 4 to 5 disks on TrueNAS 24.10.0.2. The expansion speed was ~55 M/s but I got it to ~110 M/s by increasing the raidz_expand_max_copy_bytes parameter: admin@truenas ~ $ cat /sys/module/zfs/parameters/raidz_expand_max_copy_bytes 167772160 admin@truenas ~ $ echo $((10 * 167772160)) | sudo tee /sys/module/zfs/parameters/raidz_expand_max_copy_bytes 1677721600

The OP was able to double is expansion speed…