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HoLB and VOQ


In another attempt to get some MDS switches for our extended SAN, I have veto'ed using Tape drives between data centers until Brocade can tell me exactly how they have "got around Head of Line Blocking (HoLB)" if they don't have Virtual Output Queues (VOQ). They just said that it is not a problem anymore. The senior SE that I met said that buffer credits had been extended and this would stop any issues. He also said that virtual channels could be the fix. Mind you he looked at me blankly for about 2 minutes when I asked about HoLB...

Is VOQ a Cisco only method of getting around HoLB? I read in some forum that obviously hates Cisco that VOQ has been around for years and Cisco probably pinched the idea, etc, etc...

If that is the case, then why don't other switch vendors say that they have VOQ's or similiar technology to reduce the risk of HoLB?




Re: HoLB and VOQ

Virtual output queues are nothing new. Cisco and other vendors have been using them for years in networking gear to assure performance of voice applications over data networks.

VOQ's address the issue that some data is more latency sensitive then others, and has to get "front of the line" privileges".

Most vendors us a trick of separating their port buffers into multiple virtual buffers, then using an algorithm to drain the most "important" buffers first.

The thing about VOQ is that technically you don't need it if the switch is "wire rate". Example - if a switch truly process's at wire rate, then no data will get buffered. It will just slip through the fabric with no choke points in between.

The sad thing is that no switch is "wire rate". Brocade says they are.. but that is just hogwash. If they were wire rate they wouldn't need buffers memory.. which last time I checked was all over ALL vendors switching products.

The scary thing is.. Brocade says they are "Wire Rate" so they don't have to invest in the ASICS that are capable of doing VOQ.

I call BS to Brocade on that one.

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