[RT-AC66U] Strange transmission bottleneck using builtin FTP

[Avin4]

Hi there,

I have an AC66U with latest Merlin firmware 380.69. When using the builtin FTP (FileZilla as the client, TLS support disabled) and transferring large files from the USB HDD attached to the router to my PC I spotted a strange phenomenon: that if the source file size exceeds certain amount (e.g. an ISO file of 40 GB), the transfer speed would be capped at around 5MB/s, which is way too slow even considered AC66U's USB 2.0 capacity. In the Web UI it shows CPU at this time was 100% occupied, but I used "top" command in SSH and observed it's only around 25%, which is weird! And when transferring a smaller file, e.g. a video file of 5GB, I can get 18MB/s, which I think is normal for USB 2.0 (with CPU usage at around 80% shown in SSH shell) . Can someone tell me the reason? Is there anything I can do to improve this?

Self check:
1. USB 2.0 disk (1TB 2.5' HDD in an enclosure, using only the USB cable for the power supply) is formatted into ext3, and when connecting to PC directly the reading speed is OK with no issue (WIN 10 with the help of some tool to read ext3), so I think the power supply of this enclosure is OK.
2. CAT6 GB cable runs from router to PC. I have other PCs on the same LAN and checked file copy through LAN, output could achieve over 900 Mbps from one PC to another, so the cables are good.
3. Disabled QoS, no use. Besides even when QoS is enabled I can get normal transfer speed when the source file is smaller, say 5GB. So I think QoS is not the cause.
4. Switched to SMB and the situation is similar, i.e. normal speed when copying the 5GB video and slow when copying the 40GB ISO.
5. Checked the disk using firmware, clean. Rebooted both router and PC, the same.

CPU usage in WebUI

FTP speed transferring an ISO of 40 GB from router attached USB HDD to PC

CPU usage shown in ssh shell using top command when transferring ISO

FTP speed transferring a video of 5 GB from router attached USB HDD to PC

CPU usage shown in ssh shell using top command when transferring video file

 

[Avin4]

Anyone can help me?

 

[joegreat]

Anyone can help me?

Yes, if you can explain in 1 (one) sentence your problem! The posting above is to fuzzy to understand what is your issue...

 

[ColinTaylor]

I don't know the answer to your question but I'm not seeing the same thing here. But I'm using John's fork instead of Merlin's if that makes a difference. I can transfer a 2GB file and a 9GB file and they both transfer at exactly the same speed, 26MiB/s.

In the Web UI it shows CPU at this time was 100% occupied, but I used "top" command in SSH and observed it's only around 25%, which is weird!

You are misreading the numbers. Look at the 2nd line from the top, in both cases the idle time is 0.0%. Almost all the CPU is being consumed by sys, io and sirq, as expected.

Perhaps Merlin uses a different version of vsftpd?

 

[Avin4]

I don't know the answer to your question but I'm not seeing the same thing here. But I'm using John's fork instead of Merlin's if that makes a difference. I can transfer a 2GB file and a 9GB file and they both transfer at exactly the same speed, 26MiB/s.


You are misreading the numbers. Look at the 2nd line from the top, in both cases the idle time is 0.0%. Almost all the CPU is being consumed by sys, io and sirq, as expected.

Perhaps Merlin uses a different version of vsftpd?

Thx for the reply! My mistake, now I understand the results correctly. So seems when transferring the 40GB ISO, more CPU is used by disk io and less by sirq, and what is the reason? Disk cache being consumed up?

You mentioned you can obtain same speed for ISO and video file, have you tried a larger one, say 20GB like mine? I think maybe this is the cause. I just want to figure out if this is due to my USB HDD or a bug hidden in Merlin firmware.
Besides this I'm very satisfied with my RC66U on Merlin. OpenVPN with GB LAN, IPTV support...

 

[ColinTaylor]

I didn't try anything more than 9GB because that's the biggest file I had lying around. I don't think I can offer any more meaningful testing as I have different firmware/CPU/RAM.

One thing that can sometimes make a difference is the underlying format of the USB drive, NTFS vs. ext4 etc. It's quite possible that the issue is with the disk driver rather than the ftp service per se.

 

[Avin4]

Well actually I have considered if the disk format is the reason. But I checked the compatibility of RC66U on Asus website, ext4 is blank... so I did not try my luck on it. As for NTFS, I need write ability hence it is not an option neither. Maybe this is the best I can get on this router, luckily I don't have many ISOs of this size

 

[ColinTaylor]

Sorry, I forgot you were using an RT-AC66U. You're correct it doesn't support ext4. You also said you were using ext3, which is probably the best choice.