Erek Dyskant on 14 May 2008 10:26:10 -0700

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Re: [PLUG] real-time data replication over highspeed WAN link

	The main worry with x86 SANs is that you're reliant on the system's
motherboard and the full linux stack for access to each SAN node,
whereas hardware SAN arrays can have a level of controller redundancy.
You'll have a DR array, but probably won't have the machines configured
to automatically fail over to the remote DR array if the local array
fails, plus a SQL server accessing its disk over a WAN link would
probably come crashing down.
	If you accept that there's the possibility for downtime if your box
goes down, or you come up with redundant x86 cluster as part of your SAN
design, then x86 SANs can be an excellent choice.  DRBD (at least the
free one) is limited to synchronizing two nodes, so you can't do a
((local array <> local array)  <> remote array) scheme.
	Another strategy is to do two separate iscsi arrays, and create two
LUNs for every machine.  Then use software raid 1 to join the two LUNs
so every machine can run if either of the arrays is down.
	Also, while you're looking at Linux + DRBD, be sure to give some
thought to OpenSolaris + ZFS + StorageTek AVS, which is also an
excellent system with a lot more flexibility (and some definite
downsides too)
	Hope this helps.


On Tue, 2008-05-13 at 21:40 -0400, W. Chris Shank wrote:
> I'm wondering if anyone has used any data replication tools like DRDB
> for high availability fail-over through a WAN link. I am considering
> the Dell EqualLogic iSCSI SAN with it's peer replication technology.
> But so far it seems like a pretty hefty price-tag for something I
> could accomplish with Linux, DRDB, and iSCSI tools. Does anyone have a
> failover site with real-time replication? If so, what are you using?
> My environment is mostly Windows Servers and MS SQL Databases - so I
> was only considering Linux because it seems like I could replace the
> EqualLogics with a Linux SAN. But I'll give consideration to other
> hardware or software solutions as well. 
> My biggest concern is if I can keep the databases in sync in real-time
> (if not real-time - how close), what bandwidth will I need to have
> been datacenters, and will the replication give me a performance hit
> at my primary location. Your suggestions are appreciated.
> -- 
> -- 
> W. Chris Shank 
> ACE Technology Group 
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