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April 8th, 2021 05:00
S4820T DCB, CAM and iSCSI
Hello, I have a pair of stacked S4820T. Until the moment we have been using iSCSI with Dell recomendations. That's it:
- Iscsi enabled
- Flow control enabled
- Jumbo frames
My cam now is like this:
cam-acl l2acl 2 ipv4acl 2 ipv6acl 0 ipv4qos 2 l2qos 2 l2pt 0 ipmacacl 0 vman-qos 0 ecfmacl 0 iscsioptacl 2 ipv4pbr 2 nlbclusteracl 1
Now I need to enable DCB on some ports for RDMA access (ROCEv2) of Hyperconverged HCI nodes.
1. Can I have ISCSI and flow control on some ports and DCB enabled on other ports?
2. I have a lot of confusion with the CAM requirements. I read in the configuration guide that I have to enable etsacl in the cam for DCB/ETS: cam-acl l2acl 2 ipv4acl 2 ipv6acl 0 ipv4qos 0 l2qos 0 l2pt 0 ipmacacl 0 vman-qos 0 fcoeacl 2 etsacl 3.
But the iSCSI chapter with DCB talks about aplying the flash:/iSCSI_DCB_Config and in this config the CAM command is different without etsacl: cam-acl l2acl 4 ipv4acl 4 ipv6acl 0 ipv4qos 2 l2qos 1 l2pt 0 ipmacacl 0 vman-qos 0 ecfmacl 0 fcoeacl 0 iscsioptacl 2
Thanks
DELL-Josh Cr
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April 8th, 2021 11:00
Hi XaviSP,
You may be better off talking to phone support for this issue, it is going to depend on your environment and how much traffic you have for which CAM profile is going to be better.
DELL-Tim G
4 Apprentice
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73 Posts
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April 9th, 2021 07:00
Hi,
please note DCB with PFC and LLFC (link level flow control) are mutually exclusive.
A switch can either do classic link level or priority based flow control (PFC).
Once you enable DCB the switch will no longer honor classic pause frames which in fact disables flow control for non-DCB-capabable (iSCSI) devices.
Either configure all required devices to use DCB/PFC or leave out ethernet level flowcontrol for your HCI nodes.
If you mix DCB and legacy devices in the same network throughput might suffer.
I don't think you need to alter the CAM profile as ETS rules are treated as system ACLs.
Tim
XaviSP
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April 11th, 2021 06:00
In fact, we have a mix of devices over the same switch: DCB devices, non DCB devices, ... What do you think is better? Enable flow control in all the switch, disable flow control, enable DCB?
Thanks
leon2602
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April 12th, 2021 00:00
A switch can either do classic link level or priority based flow control
DELL-Tim G
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73 Posts
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July 14th, 2021 03:00
There is no general recommendation or best solution.
Most iSCSI storage vendors recommend asymmetric LLFC (RX on TX off on the switch port) as it was tested to give the best results.
However this mechanism is good as long as you have a dedicated network for iSCSI.
As soon as the network is shared for traffic types LLFC will cause you headaches as it can only throttle/pause a physical link and does not distinguish between traffic types.
A solution could be to disable flow control on converged links not only carrying iSCSI and to let TCP do that job.
This will probably increase the retransmit rate for iSCSI flows but avoids the throttling of other traffic types.
Only a good DCB concept is expected to give optimal performance for converged networks.