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Occasional Contributor
Posts: 17
Registered: ‎10-09-2014
Accepted Solution

QoS Pros and Cons

[ Edited ]

I've got quite an open ended question - looking for opinions out there really.

 

If you had a totally clean slate and were starting again from scratch, would you use QoS in your SAN Fabrics?  We don't, and am not about to start using it on our existing SANs, but have been wondering whether it's something we ought to consider for the future.

 

What are the Pros and Cons?  Does it really protect live data from test data?  Is there an overhead that can cause other problems?

 

Thanks in advance!

Frequent Contributor
Posts: 141
Registered: ‎05-26-2009

Re: QoS Pros and Cons

Ages ago I wrote an article about it: https://www.ibm.com/developerworks/community/blogs/sanblog/entry/san_myths_uncovered_quality_of_service_in_brocade_fabricos2?lang=en

But meanwhile and with the info that you start from scratch, it might make sense to use it. Because with 16G platform that already has plenty of buffers built in the hardware and the command portcfgeportcredits the reservation of precious buffers doesn't play a so big role anymore. In addition there are features in the latest FOS versions like Slow Drain Device Quarantine that rely on QoS (AFAIK) and others might come in future releases.

So yeah, if you think the admins in your company will work closely and cooperatively together to determine the best priority for each flow, QoS can help you.

Occasional Contributor
Posts: 17
Registered: ‎10-09-2014

Re: QoS Pros and Cons

Thanks thats really helpful.  I guess the key thing is getting the priority of each server right - because the obvious divisions, like live vs dev or highest service level vs lower service levels, could still lead to too much in the top QoS group.  Our philosophy up to now, though we realise it has some flaws, is that it's better for everyone to be in one big group (in disk terms as well) then lots of little groups, as then there's more wiggle room for everyone.

 

Is there any overhead to using QoS?  Any sort of extra processing that needs to go on on the switch that could be detrimental?

Valued Contributor
Posts: 521
Registered: ‎03-20-2011

Re: QoS Pros and Cons

You don't have to start from scratch to start using QoS. You can start using QoS in your current environment - that's not difficult.

If you call 4 default VCs one big group, then you can't call QoS enabled environment "a lot of smaller groups", because your default big group becomes QoSM and is not getting any smaller. Additionally, you get even bigger QoSH group, and a bit smaller QoSL group. That looks like "three big groups" to me.

From what I understand, QoS is implemented in the ASIC, so it shouldn't add any overhead. Maybe only when CP parses the zoning config and pushes it into the ASIC(s)
Occasional Contributor
Posts: 17
Registered: ‎10-09-2014

Re: QoS Pros and Cons

So if you don't have QoS enabled at all, do you only get 4 VCs?

Valued Contributor
Posts: 521
Registered: ‎03-20-2011

Re: QoS Pros and Cons

yes
Occasional Contributor
Posts: 17
Registered: ‎10-09-2014

Re: QoS Pros and Cons

You'll be pleased to know we've decided to implement QOS on our SANs.

 

Is there a way to view traffic in each QOS group or VC?

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