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Replica Placement Strategy

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Posted: 6 Oct 2004
 

Here's a design scenario for replica placement contributed by a reader. We've included some advice from one of our Forum experts as well, in hopes this will trigger some thoughts about how you consider placing replicas in your environment.


Reader:

We have Site_A and Site_B connected trough a 1.5Mbit WAN. On Site_A we have three servers; on Site_B we have one server. eDirectory is divided into three partitions: [ROOT], [O=SITE_A] and [O=SITE_B].

How would you place the replicas? This is my idea:

Partition [ROOT]:

- Master on SITE_A on Server1
- R/W on SITE_A on Server2
- R/W on SITE_B on Server1

Partition [SITE_A]:

- Master on SITE_A on Server1
- R/W on SITE_A on Server2
- R/W on SITE_A on Server3
- Reference on SITE_B on Server1

Partition [SITE_B]:

- Master on SITE_B on Server1
- R/W on SITE_A on Server1
- R/W on SITE_A on Server2

Here's an Overview of the Services:

SITE_A:

- 500 Users, 200 Workstations
- Server1: FileServer, ZFD4, DNS/DHCP, GroupWise, Backup
- Server2: FileServer, NDPS
- Server3: BorderManager

SITE_B:

- 200 Users, 100 Workstations
- Server1: FileServer, ZFD4, DNS/DHCP, NDPS, Backup

Users on SITE_B are using GroupWise on SITE_A. Inventory Server for ZFD4 is located on SITE_A. Internet Access is also provided through SITE_A. At the moment, the Master-Replica of SITE_B is located on Server1 at SITE_A. Server1 on SITE_B just has a R/W.

Forum expert:

This looks like a good design. You're trying to limit WAN utilization, which is good. Depending on your tree design, you may not need a replica of root at Site_B. You may be able to get away with all replicas of Root at Site_A.

It's also a good idea to design with disaster recovery in mind. Considering that, you really should place a replica of every partition at both sites. That way you may lose an entire site and still be able to restore the whole tree.

Personally, I like to have all masters on one server closest to where I am, and then a set of R/W on another server placed on another site. However this requires you to use MLA licensing, since MLA licensing doesn't need access to the master replica in order to obtain licenses. If you're not using MLA licenses, it's better to have the master replicas placed near the users who are acquiring licenses.

Your design is good in theory. In practice, depending on the services/resources running on the different servers, you might need to add replicas to server3 in Site_A as well. This is hard to tell without knowing how your users work with the various resources on your servers.


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