On Wed, 2010-09-15 at 14:25 +0200, Lars Ellenberg wrote: > On Wed, Sep 15, 2010 at 02:17:09AM -0400, Peter Larsen wrote: > > Never in an active/active configuration. drbd makes a lot of sense if > > you have a single active host but with an active/active solution you > > have no way of dealing with distributed conflicts? drbd starts by > > keeping two copies of everything on separate nodes and asyncronly > > Nope. For obvious reasons, > we allow dual-primary mode only in _synchronous_ replication mode.
Wasn't aware that DRBD allowed for synchronous replication. I'm still trying to grasp how multiple local volumes being updated across a network helps? Not only do you have 3 IOs for each logical IO; you now have journals too? I would like to learn a bit more about the architecture being attempted with sync replication mode here - so I'll start looking for some docs. I presume http://www.drbd.org/docs/about/ is the place to go to learn the details? > > exchange the data between the nodes to keep the logical device > > consistent. For an application that runs on two separate nodes > > independently this just makes no sense. Not only are we tripling the > > writes needed but you have no way to dealing with two master copies in > > conflict. > > But yes, dual-primary is more complex to handle, as most replication link > breakage currently necessitates an immediate reboot of one of the servers. > So you better make sure your replication link is redundant and stable in > itself. We try to improve the handling. No kidding! It's a lot more complex - you have chosen a very impressive challenge! I'm trying to hold DRBD up against the complexity of a NAS/SAN configuration. Would you have any thoughts on comparing the "decentralized storage" approach of DRBD with a centralized storage like a NAS? > > My main concern is the huge overhead with the solution. drbd looks like > > the way to go in passive/active solutions though. I'm still partial to that opinion. But I also should disclose that I just learned about drbd a week ago. I wouldn't exactly call myself a expert in that architecture ( ...... yet). -- Best Regards Peter Larsen Wise words of the day: I could dance with you till the cows come home. On second thought, I'd rather dance with the cows till you come home. -- Groucho Marx
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