Implementing lagged copies or not?
Hi All! I wonder if you want to give me some input on this subject. We are planning to implement a HA solution at a customer. The Budget allows for 2 cas servers and 3 mbx servers. The mbx servers will be added to a dag. Our plan is to have 4 databases with 1 passive copy and one lagged copy balanced on the 3 servers. My first question is if should spread the copies to balance the user load or if we should dedicate one of the servers to hold the lagged copies. Question number 2, should the volumes that stores the lagged copies be included in the backup schedule? Question nr 3, should we use lagged copies at all? Best regards, Uc S
February 25th, 2011 3:24pm

My question to you is why you think you might want a lagged copy? Are you going to do a backup at any stage? If you are not then you might do well to have an Active, a standby and then a 2-hour (whatever, I dont care) lag. If you are going to do some sort of backup then the lag is pretty pointless unless you are planning on having a situation where you might need to replay the lag. Any use case for lagging is marginal at best. "uc2be" wrote in message news:e82bc319-3b33-47c2-a28f-b065a08f5a2b... Hi All! I wonder if you want to give me some input on this subject. We are planning to implement a HA solution at a customer. The Budget allows for 2 cas servers and 3 mbx servers. The mbx servers will be added to a dag. Our plan is to have 4 databases with 1 passive copy and one lagged copy balanced on the 3 servers. My first question is if should spread the copies to balance the user load or if we should dedicate one of the servers to hold the lagged copies. Question number 2, should the volumes that stores the lagged copies be included in the backup schedule? Question nr 3, should we use lagged copies at all? Best regards, Uc S Mark Arnold, Exchange MVP.
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February 25th, 2011 3:55pm

uc2be, Are the servers all located in a single site or is there a WAN between the servers? What are the business requirements towards high-availability / backup / DR? For me it would - amongst other - heavily depend on the existing network infrastructure (WAN/LAN/latency etc). Possible scenario When I need to set up a DAG, I tend to balance the load between members of the DAG. Usually, when 2 members exist, I have 50% of the users active on Server 1 ander 50% active on Server 2. Both of these servers hold than a passive copy of the other server's database. However; when you add a third node. This brings up other opportunities. You can still load-balance between the first two Servers AND you could use e.g. the third server to host lagged copies of the other 2 server's databases. Obviously that all depends on what you are trying to do (is there a need for lagged copies?). Another use of that 3rd server an could be for hosting passive copies and than to take backups from. You'll need a 3rd-party backup software to do that. But keep in mind that there are some elements you need to take into account when backing up passive copies (with regards to restores). For that matter, take a look here: http://blogs.technet.com/b/ehlro/archive/2010/02/13/backup-issues-and-limitations-with-exchange-2010-and-dag.aspx?wa=wsignin1.0 edit: rephrased
February 25th, 2011 3:58pm

Hi and thanks for your answers. Some more information on our environment. All servers are located in the samt ad site and data center. The SLA is not signed yet so I have no detailed info about the required uptime. So for now this is more of design decision that I have to make before I present the solution to the customer. I though that we should use lagged copies with a lag of at least 7 days to protect our self from logic corruption, I know that this does not happen very often to day, but stranger things have happened :) Even if we take backup this still not protects against corruption ,right? Thanks, UC
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February 25th, 2011 4:29pm

uc2be, backups also protect against corruption (up till a certain degree) as do lagged copies for that matter. Lagged copies allow you to decrease the need for backups (although I would never recommend you not using any backups at all) It's as Mark said, there are not so many use case for lagged copies (only if required). Grts, Michael
February 25th, 2011 4:34pm

FWIW, Microsoft IT considered using lagged copies for their own Exchange infrastructure and ultimately decided not to use them internally.
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February 25th, 2011 7:01pm

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