# Jumbo frames and windows mtu size



## Sylhouette (Oct 5, 2011)

Hello all.

I am trying to use jumbo frames between my zfs iscsi target and my windows 2003 server

Now i only have two mtu sizes on windows 4088 and 9014

When i set my mtu size on my FreeBSD to the same, it feels like something is wrong.

is windows mtu 9014 the same as 9000 on FreeBSD, if so what is the correct mtu size for 4088 then, it is not 4000, because if i set that, copies to the zfs iscsi target takes more time than the default 1500 value.


regards
Johan Hendriks


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## yongari@ (Oct 5, 2011)

Have no experience on Windows server but I believe the meaning of MTU should be the same. Jumbo frame support highly depends on controller and driver quality. Even though both parties use different MTU value, TCP will use advertised MSS so there should be no problem in general.

If you use non-server class controllers you'd better to stick on standard MTU. Server class controllers have better DMA engine and optimized for efficient packet processing for all frame size. Non-server class controllers have several limitations for certain frame size and generally they are tuned for standard MTU. I would choose best MTU value based on experimentation.

For server class controllers, it's normal to see 990Mbps with jumbo frame for bulk TCP transfers. For non-server class controllers, the number would be 940-960Mbps with jumbo frame, so bottle-neck wouldn't be ethernet.


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## Sylhouette (Oct 5, 2011)

Thanks for your answer.

The nics are two intel desktop cards.
The performance with the default mtu size, is more than acceptable, so i leave it like it is.

Thanks again.

regards,
Johan


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## RusDyr (Nov 7, 2011)

"it feels like something is wrong" actually very unspecified problem.  I would recommend to check actual size in iSCSI traffic by tcpdump, turn on path mtu discovery temporary.


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## Savagedlight (Nov 9, 2011)

Sylhouette said:
			
		

> Hello all.
> Now i only have two mtu sizes on windows 4088 and 9014
> When i set my mtu size on my FreeBSD to the same, it feels like something is wrong.
> 
> is windows mtu 9014 the same as 9000 on FreeBSD, if so what is the correct mtu size for 4088 then, it is not 4000, because if i set that, copies to the zfs iscsi target takes more time than the default 1500 value.


Basically, the Windows setting includes the header bytes and the FreeBSD setting does not. If you set it to 9014 on Windows, your FreeBSD setting should be 9000. If you set it to 4088 on Windows, your FreeBSD setting should be (4088-14)=4074.


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