Why my raidz2 vdev has very slow read speed at some specific situations?

[Kaname]

My TrueNAS Scale Hardware Configuration:
Epyc 7302 + H12SSL-i + 16G*8 DDR4 RECC 2133MHz
SSD_Pool：2*WD SN640 7.68TB
HDD_Pool：8*WD HC550 16TB (add 2*Samsung 970evoplus 2T as metadata vdev)
10G sfp+ NIC (connect to switch)

Now:
I use notebook with 2.5Gbps port connected to the switch
①when I write something from notebook to truenas, it works well with 270MB/s.
②when I read something from HDD_Pool to notebook, it only has 30MB/s.
③however, when I read the samething from SDD_Pool to notebook, it can be 270MB/s.
④even stranger, when I copy something from HDD_Pool to SSD_Pool, it can be 380MB/s.

Question:
Who can explain the reason of ② and give some advices to solve it?

 

[Davvo]

Is this related to specific (small) files size?
Please provide more info about your NICs.

Please read the following resource.

Resource - High Speed Networking Tuning to maximize your 10G, 25G, 40G networks

Both FreeBSD and Linux come by default highly optimized for classic 1Gbps ethernet. This is by far the most commonly deployed networking for both clients and servers, and a lot of research has been done to tune performance especially for local...

www.truenas.com