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#1 2008-05-06 19:43:06

Levis
Member
Registered: 2008-05-06
Posts: 40

MTU 1500 or 9000?

I've done some tests, with 2x500gb JBOD.
Changing MTU from Windows XP and with ifconfig for DNS.


Reading 1 file of 775 MB from NAS DNS-323.

XP  MTU 1500
NAS MTU 1500
50s - 15,5 MB/s

XP  MTU 1500
NAS MTU 9000
46s - 16,8 MB/s

XP  MTU 9000
NAS MTU 1500
43s - 18 MB/s

XP  MTU 9000
NAS MTU 9000
36s - 21,5 MB/s


Reading 1 file of 4536 MB from NAS DNS-323.

XP  MTU 1500
NAS MTU 1500
295s - 15,4 MB/s - CPU Average Level for Samba 95%

XP  MTU 9000
NAS MTU 9000
218s - 20,8 MB/s - CPU Average Level for Samba 66%



Writing 1 file of 1048 MB to NAS DNS-323.

XP  MTU 1500
NAS MTU 1500
67s - 15,6 MB/s - CPU Average Level for Samba 90%

XP  MTU 9000
NAS MTU 9000
56s - 18,7 MB/s - CPU Average Level for Samba 65%


The CPU Level is lower with MTU 9000 according to:
http://en.wikipedia.org/wiki/Jumbo_Frame
http://sd.wareonearth.com/~phil/jumbo.html


So, do you think it's good to leave it to 9000 or not?

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#2 2008-05-06 20:06:34

sjmac
Member
Registered: 2008-01-21
Posts: 222

Re: MTU 1500 or 9000?

It depends. Do you like debugging wierd network related breakages? Do you understand all the stuff you've read about why jumbo frames are useful, and why they aren't appropriate for most network configurations?

I don't really have a strong understanding of the issues, but I think that any non-jumbo aware switches or hubs on your network will break things in an obvious way. Any non-jumbo devices connected directly to your network might break things in a partial/non-obvious way as packet fragmentation happens, or packets get dropped.

Personally I tested with an mtu of 7500 because my DNS323 wouldn't take 9000 - after I set the value and then queried it again the value set was always a little bit higher at 9004:

Code:

/ # ifconfig egiga0
egiga0    Link encap:Ethernet  HWaddr 00:19:5B:EF:A7:F7
          inet addr:192.168.21.107  Bcast:192.168.21.255  Mask:255.255.255.0
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:125176 errors:0 dropped:0 overruns:0 frame:0
          TX packets:218567 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:512
          RX bytes:21607204 (20.6 MiB)  TX bytes:285044096 (271.8 MiB)
          Interrupt:21

/ # ifconfig egiga0 mtu 9000
/ # ifconfig egiga0
egiga0    Link encap:Ethernet  HWaddr 00:19:5B:EF:A7:F7
          inet addr:192.168.21.107  Bcast:192.168.21.255  Mask:255.255.255.0
          UP BROADCAST RUNNING MULTICAST  MTU:9004  Metric:1
          RX packets:125240 errors:0 dropped:0 overruns:0 frame:0
          TX packets:218618 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:512
          RX bytes:21611764 (20.6 MiB)  TX bytes:285049291 (271.8 MiB)
          Interrupt:21

/ # ifconfig egiga0 mtu 7500
/ # ifconfig egiga0
egiga0    Link encap:Ethernet  HWaddr 00:19:5B:EF:A7:F7
          inet addr:192.168.21.107  Bcast:192.168.21.255  Mask:255.255.255.0
          UP BROADCAST RUNNING MULTICAST  MTU:7500  Metric:1
          RX packets:125277 errors:0 dropped:0 overruns:0 frame:0
          TX packets:218642 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:512
          RX bytes:21614206 (20.6 MiB)  TX bytes:285051163 (271.8 MiB)
          Interrupt:21

/ # ifconfig egiga0 mtu 1500
/ # ifconfig egiga0
egiga0    Link encap:Ethernet  HWaddr 00:19:5B:EF:A7:F7
          inet addr:192.168.21.107  Bcast:192.168.21.255  Mask:255.255.255.0
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:125430 errors:0 dropped:0 overruns:0 frame:0
          TX packets:218673 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:512
          RX bytes:21667392 (20.6 MiB)  TX bytes:285055349 (271.8 MiB)
          Interrupt:21

I don't know why it is different for 9000 (and my switch says that it is good up to over 9100), but it made me nervous.

In the end I set back to 1500 so that devices on my network worked properly :-)

Last edited by sjmac (2008-05-19 23:49:39)

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#3 2008-05-06 20:14:02

sjmac
Member
Registered: 2008-01-21
Posts: 222

Re: MTU 1500 or 9000?

OK, I promise I won't post this pic again, but just so that it is finally in a relevant thread, and because the results are slightly different to yours ...

This picture is writing a single 1GB file to the DNS323 from a Vista (32 bit) machine with an Intel PRO/1000 PT PCI Express NIC.

This was done before the Vista SP1 was released/installed, but on this particular machine SP1 only improved the read speed (in to the Vista machine).

http://tenuouslink.net/blogs/gadgets/uploaded_images/jumbo-speeds-annotated.jpg

Last edited by sjmac (2008-05-19 23:43:39)

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#4 2008-05-06 20:39:07

Levis
Member
Registered: 2008-05-06
Posts: 40

Re: MTU 1500 or 9000?

I've read some topic about jumbo frame but there's no specific post about the trouble.
I admit that I'm not particular expert with these setting so I'm doing "some practice" tongue

The differences between your graph and my data is for DNS323 & PC with 1500/7500 MTU, from your graph it seems that a configuration with different MTUs has a lower performance, instead in my tests it's in the middle.

But from Windows XP with my Marvell Yukon I can't change MTU to 7500, only 4088 and 9014 byte settings are available, so I tried with 9000.
Also my DNS switch to 9004.

I thought that it was quite the same tongue but I have no packet dropped.

In my network I have my pc connected to a 3com gigabit switch attached to NAS and to ADSL modem.
So the problem could be the ADSL modem from my ISP that probably it's configured as MTU 1500?

Any other advice would be interesting big_smile tnx

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#5 2008-05-06 22:59:12

sjmac
Member
Registered: 2008-01-21
Posts: 222

Re: MTU 1500 or 9000?

Levis wrote:

But from Windows XP with my Marvell Yukon I can't change MTU to 7500, only 4088 and 9014 byte settings are available, so I tried with 9000.

That's fine, I think I had my Windows machine set to 9014 ...

In my network I have my pc connected to a 3com gigabit switch attached to NAS and to ADSL modem.
So the problem could be the ADSL modem from my ISP that probably it's configured as MTU 1500?

*Should* be OK - Your ADSL modem is probably built in to some device that is also a router, and it is the routers job to fragment packets sensibly. (Switches are a problem though, because they don't fragment packets, they just silently drop them).

Most computers now use path MTU discovery and maybe even black hole detection so that each network connection made is supposed to get configured with optimum parameters. In the past I've used an ADSL router has a wierd bug that stops that working though, and I had to set the router MTU to 1492 (or similar). nowdays I prefer to keep everything at default settings as they are more likely to have had some testing by the vendors ;-)

I think unless you are an expert with the right tools (and I'm not) troubleshooting network problems is some times black magic and superstition. And the interaction between different values that you "tweak" could be unexpected!

Have fun :-)

Last edited by sjmac (2008-05-06 22:59:43)

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