2 x 1GbE Links But Only 1GbE Writes

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2 x 1GbE Links But Only 1GbE Writes

RS1221+, RS819, RS217
Operating system
  1. macOS
Mobile operating system
  1. iOS
Last edited:
I'm scratching my head on this one as since my last actual test we have had DSM updates, macOS updates and probably a switch update.

The NAS is my RS217, so weak in just about every area, running with 2 x 1GbE links (it only has 2 ethernet ports), no LAG, no Bond, large MTU, SMB3. Volume is a single SSD and no errors or issues reported anywhere. I noticed that it seemed to be running slower on transfers so ran a few tests and double-checked network settings.

For read speed I am getting around 230MB/s (slightly up on last year) but only peaking at around 130MB/s and typically around 115MB/s for writes - close to 1GbE speed. It used to be around 200MB/s for writes and no lower than 180MB/s. Data throughput is being shared equally between the 2 x 1GbE links.

As an example using a typical speedtest app:

Server Message Block: SMB : Apple M1.png

DSM7 Resources - not exhausting the (anaemic) CPU or RAM:

 2022-01-14 at 15.52.09.png

DSM7 Settings:

 2022-01-14 at 15.02.48.png

 2022-01-14 at 16.26.14.png

My switch port settings (port 17&18 being used for this NAS):

 2022-01-14 at 15.04.05.png

NAS SMB config:

root@Dragon:~# cat /etc/samba/smb.conf
# Copyright (c) 2000-2019 Synology Inc. All rights reserved.
## Don't Panic, in big friendly letters ##

    server multi channel support = yes
    aio read size = 0
    aio write size = 0
    printcap name=cups
    winbind enum groups=yes
    encrypt passwords=yes
    min protocol=SMB2_10
    local master=no
    syno sync dctime=no
    passdb backend=smbpasswd
    max protocol=SMB3
    winbind enum users=yes
    load printers=yes

Can anybody clue me in as to why the link only achieves around 1GbE speeds for writes or spot an error anywhere?

Amongst other things yes, previous tests were under DSM6 but reasonably sure I tested it under DSM7 beta but didn't keep a record so I guess it was unremarkable.

There is 2Gbps of bandwidth available (less overheads) and I am achieving that in one direction - 231MB/s or 1.85Gbps in the example above - but only 131MB/s or 1.048Gbps in the other direction.

I'm wondering about the CPU again as if I lower the MTU to 1500 the write speed drops to 90MB/s, suggesting that it is a packet limitation.
@AdrianEarnshaw - Thanks and it looks like you are on to something as the online data sheet does say that... now.

It used to say 171.7MB/s, which is around what I could get with spinners whilst achieving the figures in the first post with SSDs. I wonder what Synology did that reduced the maximum write speed by such an amount.

The original datasheet:

 2022-01-15 at 16.59.22.png


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