SAN Myths Uncovered 2: The LD mode (Brocade)
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I didn't blog for a while now because of an internal project. Like each software development project it's never really over and development will be going on in the next years to bring in new functions, but I hope I have some more time for blogging again now. :o) I also decided to go a bit away from the long blog posts I did in the past to more conveniently readable short posts if possible.
Long distance modes
Brocade has basically 3 long distance modes:
So what's the problem with LD?
If you have two data centers with a distance of 30 km between them and you configure 60 km, the switch will only assign the buffers for the measured 30 km. Increasing the desired distance doesn't change anything.
Wait! Why should I increase it anyway?
As written above the number of buffers depends on the distance. The switch just calculates the amount of buffers by the number of full sized frames (frames with maximum frame size - usually 2kB) needed to span the distance. But the problem is: in real life the average frame size is actually much smaller than the maximum one.
In the picture above you see a write I/O out of a fibre channel trace. The lines with the rose background are the frames from the host, the ones with the gray background are the responses from the storage. The last column shows the size of the frame. Only the 4 data frames have the full frame size. The other 3 frames have a size far smaller than 2kB. So the average frame size in this example is just 1.2kB. With this average frame size you would need almost double the amount of buffers to fill the link than the number the switch calculated! And it could be much worse. I ran a report over the full trace and the average frame size for the transmit and receive traffic was:
Given that numbers and added a "little buffer reserve" you would need 3 times the buffers than the switch would use!
Okay so let's give it more buffers!
Yes, for LS mode this would exactly be the action plan. But remember: For LD mode, the switch just uses the measured distance. The desired distance is only used as an additional maximum. So if you have 30 km and configure 20 km, it will only assign the buffers for 20 km. If you configure 50 km, it will only assign the buffers for 30 km. So my general recommendation is:
Use LS instead of LD!
LS mode gives you the full control. And use it with enough buffers by configuring a multiple of the physical distance. 3x is a good practice but you can increase it even more if there are buffers left. You can always check the available buffers with the command "portbuffershow".
Don't leave those lazy buffers unassigned but use them to fill your links!