SRDF over what media?
by Jesse on Feb.06, 2007, under Best Practices, Data Migration, Replication
Well – Tomorrow I should have data replication going between the two Symms. And it gives me pause.
We’re going to be using SRDF/A for our replication. To those who are not familiar with the EMC terminology, SRDF/A is a “Semi-Asychronous” form of SRDF that provides consistency points in the data being transmitted without affecting production performance.Â
SRDF inserts a “Checkpoint” periodically into asyncronous traffic. The target frame will only write a block of changes, called a “Delta-Set”, when the ending checkpoint has been received. If a link fails before the checkpoint is received, the previous block of data is considered to be invalid and discarded.
This allows a recovery-point of 10-15 minutes, with guaranteed consistency, over a longer distance. (Our planned replication distance is approximately 1500 miles)
The other option, albeit too expensive for the bean-counters who manage our money, is multi-hop SRDF, which allows you to replicate to a bunker site 10-15km away from the primary site in full synchronous mode, and then from the bunker site to the DR site in Async. or SRDF/A mode. This allows a recovery up to the point of failure in the event the primary site is lost, and recovery to the last delta-set in the event of both a primary and bunker site loss. (nuclear explosion?)
So the options for distance are: Ethernet, and ethernet. The longest peice of dark fibre I’ve ever seen covers the 35km or so between capitol hill and and the congressional DR facility. They ran full Syncronous mode but the users never noticed because they never saw what performance was like without the 30ms round-trip.
The Symmetrix supports three protocols for SRDF
*Â IP (current max 1gb per link)
*Â FibreChannel (current max 4gb per link with DMX-3 and 5772 code)
*Â Escon (The original standard for SRDF going all the way back to the Symm3)
FC and Escon are good for limited distances, With long-wave (1300nm) optics you can do about 10km reliably. With a good DWDM set you can stretch that out significantly, (not native, the DWDM hardware acts as a repeater of sorts) plus will allow you to put multiple links down the same fibre-pair.
Ethernet seems to be the most often implemented version these days. I’ve seen a few, though not many, “Symmetrix RFA (RDF over Fibre) –> Nishan IPS3300 –> Ethernet –> Nishan IPS3300 –> Symmetrix RFA” types of implementations, but it seems to me that you’re just throwing that many more potential breaks in the transmission line, plus every time you have to decode a signal and re-encode it in another format you’re losing a step.Â
(Even the fastest computer hardware takes time to process data, nothing hands it straight across.)
Of course, then the bean-counters get into it….the RE (RDF/Ethernet) adapters are more expensive than simply dedicating two ports of your existing FA (Fibre Host Adapters) to the RDF functionality.
Just ranting. 
February 6th, 2007 on 2:46 pm
But sir, you wield the mightiest axe of them all! When you get pushback from the bean-counters, you say “My friends Mr. Sarbanes and Mr. Oxley would like to have a meeting with you in 20 minutes.”
February 7th, 2007 on 3:09 am
LOL – I keep that up. Telling my boss that the new DMX3 with 72 code has RSA encryption built into it really didn’t have the effect I’d hoped either.
Instead of asking me to put a proposal together (since we’re already over 70% in our production Symm) for moving production to a DMX3 and moving the current DMX to our DR site as a second target) he told me it probably wouldn’t happen in my lifetime.
Oh well – so much for getting to play with the new toys. And we still have unencrypted data on our disks.