Zyxel NSA325 slow drive access, high CPU in interrupt

Linux is running great, but it's limited on read/write speed. Seems both HDD are limited to ~30MB/s when using basic tools (and so is flash drive). NFS is limited to 30MB/s. cp is limited to 30 MB/s. pv is limited to 50MB/s. dd is slow as well (unless we use oflag=direct!)

However hdparm is much faster, the only tool that gives real speeds (alongside dd oflag=direct)

(dmesg is large and is attached).

root@debian:~# uname -a
Linux debian 6.14.6-kirkwood-tld-1 #1 PREEMPT Wed May 14 17:08:50 PDT 2025 armv5tel GNU/Linux
root@debian:~# cat /etc/debian_version
12.2
root@debian:~# hdparm -Tt /dev/sda

/dev/sda:
 Timing cached reads:   556 MB in  2.00 seconds = 277.36 MB/sec
 Timing buffered disk reads: 374 MB in  3.00 seconds = 124.50 MB/sec



root@debian:~# dd if=/dev/zero of=/dev/sda bs=10M count=100  oflag=direct
100+0 records in
100+0 records out
1048576000 bytes (1.0 GB, 1000 MiB) copied, 8.14149 s, 129 MB/s

root@debian:~# dd if=/dev/zero of=/dev/sdb bs=20M
93734+0 records in
93734+0 records out
1965744455680 bytes (2.0 TB, 1.8 TiB) copied, 54092.5 s, 36.3 MB/s

<no new dmesg messages>

htop with kernel threads shows a lot of cpu spent in kworker/u4:2-events_unbound or kworker/u4:1+flush-8.16 (see attachment). However I can't tell if it's real CPU load or just CPU waiting for IO.

Wiki thread

Quote

Memory & Swap Settings

Tuning for low RAM boxes - System and HDD performance. Also read several posts after for setups for logging to RAM.

Try 65536 and increase the number gradually until 131072, it will make big difference.

vm.min_free_kbytes = 65536

But this number must be tuned to a specific box with the amount of available RAM in mind.

-bodhi
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Edited 2 time(s). Last edit at 06/12/2025 04:21PM by bodhi.

bodhi Wrote:
-------------------------------------------------------
> Wiki
> thread
>
>

Quote

Memory & Swap Settings
>
> Tuning for low RAM boxes - System and HDD
> performance. Also read several posts after for
> setups for logging to RAM.

>
> Try 65536 and increase the number gradually until
> 131072, it will make big difference.
>

> vm.min_free_kbytes = 65536
>

>
> But this number must be tuned to a specific box
> with the amount of available RAM in mind.


YOU ARE TRULY A WIZARD. This did make a difference. I'll play with it some more, but this was the answer.

Thank you!