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exchange 2010 , 2013

I was asked a question regarding DAG
as how to implement 6000 mailboxes in DAG in 3 different places of one organization which has offices in Chicago, new York and china. all 6000 mailboxes will be talking to each other?

my answer was like this which was rejected

1) to create 3 different AD sites.
2) create 6000 mailboxes in one mailbox server having mailbox database as db1 as active
create passive mailbox database -Chicago
3) do same for newyork
4) do same for china
5) create one lagged copy of database in each center

after this I couldn't answer
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1 Solution
Adam FarageEnterprise ArchCommented:
You are in the right direction, but the other question you should have asked is where are the users actually hosted?

The reason I ask this is because if I have users in each site I would have done it this way...

- Create three AD sites; one for CHI, another for NYC and the other for China
- Create three different Database Availability Groups, spanning across each datacenter (For example: NYC-DAG, CHI-DAG and CHINA-DAG)
- Each DAG would have active mailbox databases local to that site (meaning the users that sit there) split up into 200-400 users per database (e.g: mailboxes), and then create a second / third copy depending on site link (meaning if I am doing the Chicago DAG, I would have my second copy in NYC and then third copy.. or lagged in China)

That is a very basic overview on how to do this. There are other things like CAS / HUB (depending on the version, obviously Exchange 2013 does not have HUB, or a CAS Array). Some other things you should ask is..

- What is the round trip latency between each site. This is critical, as if the latency from Chicago to China is over 250ms then the DAG replication will most likely fail
- What is the RPO / RTO and backup solutions

The backup solution comes into play on if you need to actually have a lagged copy or not. If you have a backup solution running then you really dont need a lagged copy unless you want that extra layer of protection.

As for why three different DAGs, it has to do with datacenter switchovers (
pramod1Author Commented:
U have mentioned 400 users per database

That means I have to create 15 mailbox database per mailbox server , I was thinking putting 6000 mailboxes on one database and make it active

How many max users I can create on one mailbox database

Also as per u creating 15 mailbox database having 400 on each which one should I make active  and which one passive
pramod1Author Commented:
In one DAG there can be 16 mailbox servers.?

if yes, how many databases we can create on one mailbox server in DAG

Can we create 6000 mailboxes on one database or is there any limitation?

if I want to create 6000 mailboxes, can we create on one database?
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David Johnson, CD, MVPOwnerCommented:
Can we create 6000 mailboxes on one database or is there any limitation? There is no theoretical limit but there are practical limits .. The database limit is 50 where it used to be  100 don't know where you got the 16 from
Adam FarageEnterprise ArchCommented:
So as David stated, the DB limit in Exchange 2010 is 100 databases and 2013 is 50 databases.

This is going to become a complex topic, but here is my design overview of why I said what I said along with why I think throwing 6,000 users into a single database (and then simply three copies out there) is a horrible design idea.

The reason you would want to keep a  smaller database is for a few reasons, mainly reseed timing, RTO/RPO in the event you need to utilize backup software due to logical corruption (e.g: corrupt bits that are replicated across the DAG in that database. Extremely rare, but I have seen this before). The size limit of a database is technically 16TB but that is with multiple GPT partitions spanned which is really not recommended.. so with NTFS the *recommended* size limit will be 2TB.

As for the database copies, you should have a few databases per site that are active splitting across those users. Use a reasonable number, like 500 users per database. You should have three dags with one DAG per site (as discussed above). In each of these DAG per site you would have a set of active databases, a second copy of those active databases and then two other copies if you wish (or one) in the other two sites. Having multiple DAGs does a few things for this configuration:

- If I fail a database in lets say Chicago for the Chicago users, I have another server handling the traffic so the latency does not go through the roof
- If a site goes completely down (lets say China) I can properly stop that DAG and then do a datacenter switchover (see my post above, I have done this a lot)
- You are now reducing round trip RPC latency. If I have users in China attempting to use RPC / HTTPS (e.g: Outlook Anywhere) to an active database in Chicago, it may timeout thus causing user performance issues.

You wouldn't need 16 DAG members either.. it would be something closer to 6-9:

- CHI, CHINA, NY: 3 mailbox servers per site

You would then throw the active, first activated passive in the primary site and then a third (or even fourth, which is overkill in my opinion) in another site. If its between Chicago and NYC, I would recommend keeping the databases within the same continent.

Use the Exchange Server Mailbox Role Calculator to find out the best way to do this, but from an architectural standpoint this is what most would "recommend".

Exchange 2010:
Exchange 2013:

(PS: this is confusing and I dont think I did a wonderful job explaining, so ask away if you have questions)
pramod1Author Commented:
so u mean 3 different DAG.
1- NY


500mailbox on each database - lets say DB1

so for 6000 mailboxes I should have 12 databases on each mail box server.

so per site on each mailbox server there are 12 databases.

DB1,DB2, DB3, DB4, DB5, DB6, DB7, DB8, DB9, DB10, DB11, DB12.

my question is which one to make active  and which one passive. as there are 6000 active mailbox at a time
pramod1Author Commented:
u can have max. 16 mailbox servers not 16 mailbox database copies in dag?
Adam FarageEnterprise ArchCommented:
The maximum amount of mailbox servers in a DAG is 16, as that is the clustering maximum for Server 2008 (and although it changed for Server 2012, Exchange is still set at 16 members) and a maximum of 50 database copies in Exchange 2013 or 100 database copies in Exchange 2010.

To breakdown my recommendations for the topology (DAG) further..

You have three sites:


You have three mailbox servers in each DAG:

Chicago - CHIEXCH01, CHIEXCH02

Essentially there would be three DAGs here..




We will use the NYC-DAG01 as the example..

Lets say I have 3,000 users in NYC or there closest office (networking) wise is NYC. I would split those 3000 users into separate databases (about 500 per database, equaling about to about 6 databases). Those six databases should be split across NYEXCH01 and NYEXCH02, and then the server that is holding the active (which is either NYEXCH01 or NYEXCH02) the opposite in the site would contain the **first** activated passive copy. The third activated passive copy (which is our out of site member, CHIEXCH01) would be the **second** activated copy. This is the same all the way around.

The reason behind this is that I can loose the server that is hosting the active copy, and it would fail over within the site. This will not affect the users as greatly as failing over across site (as this could end up with networking latency errors). in the event that I loose all of NYC, all of the NYC databases would fail over to CHIEXCH03 so they are still available but may experience a performance degradation (depending on the networking round trip).

The same theory is applied all around. You should also apply datacenter activation coordinator to allow you to shutdown a DAG in the event a site fails without affecting the other two sites.
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