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Occasional Contributor
Posts: 6
Registered: ‎11-03-2010

Long Distance Design / Buffer Credits


 

               ,which mode should be use here LD or LS Mode ?


   desired_distance in km * (data rate / 2.125)" ...data rate = Payload

                   



              licesens ?



thanks ;)

Occasional Contributor
Posts: 6
Registered: ‎11-03-2010

Re: Long Distance Design / Buffer Credits

Sorry ....

Hello guys,

I need your help once for a Long Distance Design.My problem is I need the exact number of buffer credits for this plan.We have a Dual Fabric Design with Brocade 300 Switches and we have two ISL from every Fabric to the remote Site .The Distance of the ISL between the Brocade 300 Switches is approximately 15 km (9,32 Miles) away.My question is ,which mode should be use here LD or LS Mode ?

I know,that the best practice is LS Mode .When i use LS Mode the formula is " desired_distance in km * (data rate / 2.125)" ...data rate = Payload

How can I make a realistic statement about the payload ?Is it right, if I have less payload I need more buffer credits and if I have more payload in a frame I need fewer buffer credits?

Regular Contributor
Posts: 164
Registered: ‎05-11-2011

Re: Long Distance Design / Buffer Credits

Hi Marcel,

both modes, LS and LD are calculating the BB-Credits based on a full-size 2112k Frame.

Differences of both modes (with your 15 km):

If you configure LD with 15km the switch measures during portinitialization the real distance. If the distance is only 12km the switch will allocate only Buffers for 12km. If the distance is 17km the switch will allocate only Buffers for 15km (your desired distance).

If you configure LS with 15km, the switch will allocate Buffers for distance of 15km, independent if the distance is less then 15km.

Yes you are right: When you have smaller Frames then 2112k, you need more BB-Credits. Because you have the only possibility to configure the distance, you have to use the LS mode with 30km when you have only 1000k Frames.

Regards

Ralf

Kind Regards
Ralf
Occasional Contributor
Posts: 6
Registered: ‎11-03-2010

Re: Long Distance Design / Buffer Credits

Hey Ralf ,

Thank you for your answer,your answer was very helpful. But I have another question: how do I get my payload of a FC Frame or Frames ? How i can  find out whether the frames are smaller than 2112k?

And one more important question ,if I have a Brocade 300  with a standart POD licesens of 8 Ports and the Switch has 676 available BB credits, can i use these credits all for these 8 Ports or is this dependent on other POD licesens?


Regards

Marcel




Regular Contributor
Posts: 164
Registered: ‎05-11-2011

Re: Long Distance Design / Buffer Credits

Hi Marcel,

i have no idea how to find the Payload of your Frames. I think normally you have mostly Fullsize Frames.

Try to look in the Manual of the HBA.

Regarding the BB-Credits:

You have 676 available BB-Credits. 8 BB-Credits are reserved for each Port. 24*8=192.

676-192=484. So you have 484 BB-Credits left.

A frame spans over 0,5km if you are using speed of 8Gbps.

That means you need for 15km 30*2=60 BB-Credits minimum.

The LS or LD Mode will calculate 60+8+6=74 BB-Credits.

Regards

Ralf

Kind Regards
Ralf
Valued Contributor
Posts: 761
Registered: ‎06-11-2010

Re: Long Distance Design / Buffer Credits

Hi,

you could estimate the average frame size by dividing the number of transmitted frames in the port between the number of transmitted words (each word contains 4 bytes). Once you have the average frame size, substract the header, and the result should be the average payload size.

Each frame has a 36-byte overhead introduced by SoF, header, CRC and EoF.

Rgds

Occasional Contributor
Posts: 6
Registered: ‎11-03-2010

Re: Long Distance Design / Buffer Credits

Hey Guijarro,

Could you please give me a sample calculation with my stats:


stat_wtx            1662778881  4-byte words transmitted
stat_wrx            3360903374  4-byte words received
stat_ftx            1942352152  Frames transmitted
stat_frx            2241821737  Frames received
stat_c2_frx         0       Class 2 frames received
stat_c3_frx         2241656599  Class 3 frames received
stat_lc_rx          63999   Link control frames received
stat_mc_rx          0       Multicast frames received
stat_mc_to          0       Multicast timeouts
stat_mc_tx          0       Multicast frames transmitted
tim_rdy_pri         482     Time R_RDY high priority
tim_txcrd_z         1487728023  Time TX Credit Zero (2.5Us ticks)
tim_txcrd_z_vc  0- 3:  987222  0       1456233544  27447264
tim_txcrd_z_vc  4- 7:  4940121 20572   0       0
tim_txcrd_z_vc  8-11:  0       0       0       0
tim_txcrd_z_vc 12-15:  0       0       0       0

I think then it is easier to understand and it gives me the certainty that I have not miscalculate the result .....


Thanks ^^

Valued Contributor
Posts: 761
Registered: ‎06-11-2010

Re: Long Distance Design / Buffer Credits


please clear the stats (portstatsclear <slot/port>) and get the stats within an hour because according to the output pasted, there have been more frames tx than tx words and that means that the tx word counter has reached maximun and started over.

Occasional Contributor
Posts: 6
Registered: ‎11-03-2010

Re: Long Distance Design / Buffer Credits

stat_wtx            547077554   4-byte words transmitted
stat_wrx            2056332360  4-byte words received
stat_ftx            12235836Frames transmitted
stat_frx            14126776Frames received
stat_c2_frx         0       Class 2 frames received
stat_c3_frx         14125837Class 3 frames received
stat_lc_rx          363     Link control frames received
stat_mc_rx          0       Multicast frames received
stat_mc_to          0       Multicast timeouts
stat_mc_tx          0       Multicast frames transmitted
tim_rdy_pri         3       Time R_RDY high priority
tim_txcrd_z         9231167 Time TX Credit Zero (2.5Us ticks)
tim_txcrd_z_vc  0- 3:  0       0       9187094 3
tim_txcrd_z_vc  4- 7:  44070   0       0       0
tim_txcrd_z_vc  8-11:  0       0       0       0
tim_txcrd_z_vc 12-15:  0       0       0       0
Valued Contributor
Posts: 761
Registered: ‎06-11-2010

Re: Long Distance Design / Buffer Credits

stat_wtx = number of 4-byte word Tx since stats were cleared.

stat_wtx * 4 = number of bytes Tx since stats were cleared.

stat_ftx = number of frames Tx since stats were cleared.

(stat_wtx * 4) / stat_ftx = number of average bytes per Tx frame

(547077554 * 4) / 12235836 = 179 bytes/frame

Substracting 36 bytes of the overhead, that makes (179 - 36) = 143 bytes/payload aprox.

not a very big number indeed.

Rgds

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