[AusNOG] DCB in non-converged networks

[Mark Smith]

Hi All,

On Thu, 9 Nov 2023, 13:19 Robert Hudson, <hudrob at gmail.com> wrote:

> I largely agree with Luke.  Given you're on a dedicated iSCSI network,
> keep it simple.  DCB and other services will only add things that you'll
> later need to troubleshoot and eliminate as the root cause of network
> issues on your iSCSI network when they invariably happen (it's rare that
> I've come across a well and consistently configured iSCSI network, and I've
> been playing in that space since the mid 2000s).  Chances are your
> OS/hypervisor vendor of choice publishes best practices for how to
> configure DCB - but as noted, DCB is specifically there to deal with
> converged networks (where your iSCSI traffic is sharing an ethernet fabric
> with other traffic types), and you don't seem to have that situation.
>
> Jumbo frames help in busy iSCSI networks by increasing throughput - but
> you need to make sure every device from one end of the communications to
> the other fully supports it.  Again, follow vendor advice here.  Getting
> this wrong can cause all sorts of "fun".
>

Related to MTUs, etc., people here might be interested in a presentation on
the topic I did at NZNOG in March.

MTUs, MRUs, PMTUs
https://www.youtube.com/live/_D03z8LrauU?si=Mr5hadabWXePhCjk&t=1971



> Flow control, buffer tuning (large buffers tend to help with iSCSI
> traffic), etc, can all help to eke out a few more small percentage points
> of performance, but again, the further you drift from the KISS principle,
> the more fun you're likely to have troubleshooting later.
>

Strongly agree with KISS (foreshadowing the advice at the end of the
presentation.)



> Above all - set and document policy in all things, audit against that
> policy both at initial setup and for drift during the lifecycle of the
> environment.
>
> On Thu, 9 Nov 2023 at 12:20, Luke Iggleden <luke at iggleden.com> wrote:
>
>> Hi Andres,
>>
>> Unless you are running other services on the switch it's not useful.
>>
>> Typically these are the only useful changes:
>>
>> Jumbo Frames (YMMV), depends on vendor.
>>
>> Flow Control on (so hosts can issue back off - hopefully without dropping
>> frames)
>>
>> Depending on the switch, buffer tuning.
>>
>> Don't use control plane things, like MLAG, Stacking, STP, etc etc. Flat
>> fabric.
>>
>>
>> Cheers,
>>
>> Luke Iggleden
>>
>>
>> On 9/11/2023 11:14 am, Andres Miedzowicz wrote:
>>
>> Hello everyone,
>>
>>
>>
>> I wanted to get some opinions on the use of DCB and its associated
>> protocols in a storage-only (iSCSI), non-converged network. Any thoughts
>> about the pros and cons of enabling DCB in a scenario where 100% of the
>> traffic on a switch is bi-directional iSCSI storage (virtual machines and
>> backups)?
>>
>>
>>
>> Thanks in advance.
>>
>>
>>
>> Andres
>>
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