CentOS - Jun 2011 - Jumbo frames problem with Realtek NICs?

I was trying to do some performance testing between using iSCSI on the
host as a diskfile to a guest vs the VM guest using the iSCSI device
directly.

However, in the process of trying to establish a baseline performance
figure, I started increasing the MTU settings on the PCI-express NICs
with RTL8168B chips.

First bottleneck was discovering the max MTU allowed on these is 7K
instead of 9K but googling seems to indicate that the RTL8168B is only
capable of 4K frames.

I assumed 4K would still be better than nothing but unfortunately
bumping up the MTU to anything else but 1.5K caused the file transfers
(using NFS for easy testing), to hang at random points or more
accurate slow to a crawl.

Checking the syslog, I discovered warnings that increasing MTU with
this adapter may cause problems.

Searching around, it seems to be a common problem but there doesn't
appear to be any clear cut fix, including some suggestions to use a
third party driver.

Does anybody know of a proven solution or is the Realtek chip itself
irrevocably broken/bugged that anything above the default 1500 will
simply not work?

----- Original Message -----
> I was trying to do some performance testing between using iSCSI on the
> host as a diskfile to a guest vs the VM guest using the iSCSI device
> directly.
> 
> However, in the process of trying to establish a baseline performance
> figure, I started increasing the MTU settings on the PCI-express NICs
> with RTL8168B chips.
> 
> First bottleneck was discovering the max MTU allowed on these is 7K
> instead of 9K but googling seems to indicate that the RTL8168B is only
> capable of 4K frames.
> 
> I assumed 4K would still be better than nothing but unfortunately
> bumping up the MTU to anything else but 1.5K caused the file transfers
> (using NFS for easy testing), to hang at random points or more
> accurate slow to a crawl.
> 
> Checking the syslog, I discovered warnings that increasing MTU with
> this adapter may cause problems.
> 
> Searching around, it seems to be a common problem but there doesn't
> appear to be any clear cut fix, including some suggestions to use a
> third party driver.
> 
> Does anybody know of a proven solution or is the Realtek chip itself
> irrevocably broken/bugged that anything above the default 1500 will
> simply not work?
Realtek NICs are known to be some of the poorest interfaces available. A quality
Intel or Broadcom NIC will set you back very little in terms of cost. Just
replace it and be done. :)

--Tim

Emmanuel Noobadmin wrote:
> Does anybody know of a proven solution or is the Realtek chip itself
> irrevocably broken/bugged that anything above the default 1500 will
> simply not work?
> _______________________________________________
> CentOS mailing list
> CentOS at centos.org
> http://lists.centos.org/mailman/listinfo/centos
> 
> 
Try driver from ElRepo:
http://elrepo.org/tiki/tiki-index.php?page=kmod-r8168

Ljubomir

On Friday, June 24, 2011 01:20 AM, Emmanuel Noobadmin
wrote:
> First bottleneck was discovering the max MTU allowed on these is 7K
> instead of 9K but googling seems to indicate that the RTL8168B is only
> capable of 4K frames.
Yeah, the 8168C goes up to 7k. Some 8168B go up to 6k.

>
> I assumed 4K would still be better than nothing but unfortunately
> bumping up the MTU to anything else but 1.5K caused the file transfers
> (using NFS for easy testing), to hang at random points or more
> accurate slow to a crawl.
I have the crap enabled in Windows and it works there...but on an old 
server running OpenIndiana the driver refuses to enable jumbo frames for 
the particular chipset that the board has which is incidentally a 8168B.

>
> Checking the syslog, I discovered warnings that increasing MTU with
> this adapter may cause problems.
>
> Searching around, it seems to be a common problem but there doesn't
> appear to be any clear cut fix, including some suggestions to use a
> third party driver.
>
> Does anybody know of a proven solution or is the Realtek chip itself
> irrevocably broken/bugged that anything above the default 1500 will
> simply not work?
Using realtek's own drivers on Windows seem to be okay...not sure about 
Linux and other platforms.

On 06/23/2011 10:20 AM, Emmanuel Noobadmin wrote:
> I assumed 4K would still be better than nothing but unfortunately
> bumping up the MTU to anything else but 1.5K caused the file transfers
> (using NFS for easy testing), to hang at random points or more
> accurate slow to a crawl.
I don't know anything specifically about those cards, but you'll see 
that behavior on any card unless all of the hosts on a broadcast domain 
are using the same MTU.  You need to set all of the devices on a LAN 
segment, including the router, to the same MTU.