I have a NAS which has 2 10Gbe ports, and it provides Adaptive Load Balancing (ALB) link aggregation function.
By checking Wikipedia, it seems that in ALB, the bonding driver needs to go through complicated processes (intercept ARP, overwrite HW addresses, etc) to make the link aggregation works.
I plan to connect both network ports to a 10Gbe switch and use the ALB function to increase the bandwidth, but i worry that the ALB function will cause high CPU usage.
Will ALB link aggregation causes high CPU usage in a NAS? Will it really able to increase the bandwidth to >10Gbps without affecting the NAS performance?
Without context on the particular hardware or the OS the NAS is running, the only realistic answer in short is: It will probably improve network performance, and may or may not significantly impact CPU usage.
Whether or not it improves network performance is highly dependent on how the rest of the network is structured, how smart the switch is, and how the system will be utilized (it will have a bigger improvement if the system serves lots of connections than if it serves only a small number of connections).
Whether or not it will impact CPU usage depends mostly on how good the CPU and the network drivers are. We use ALB mode on bonded interfaces under Linux where I work, and on a reasonable entry level server CPU (a cheap fourth gen Xeon E3), it results in less than 0.5% increase in CPU utilization relative to running just one of the NIC's, but if you've got a cheap NAS with a dinky little Atom CPU, it will probably have an impact.