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 Exchange 2010 DAG A/A Vs A/P

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MadCow Posted - 04/17/2012 : 11:58:14 AM

Setting a up a test lab. Brainstorming on whether to go with A/P or A/A DAG setup.

Any thoughts/suggestions, I understand it will vary on various factors and infrastructure requirements/limitations etc.

Advise Please

Thank you

21   L A T E S T    R E P L I E S    (Newest First)
clarinathan Posted - 04/23/2012 : 11:00:02 AM
Hi Jetze,
Yes we are certainly seeing more and more of that, although it is in the high end scenarios where customers are using physical servers and lots of disks. That is where the biggest savings can be made.

Pesos Posted - 04/22/2012 : 4:00:58 PM
Jazzy Posted - 04/22/2012 : 4:00:10 PM
I'm not making this up. :)

See the comments section in the article about the Exchange 2010 Mailbox Role Calculator:
In Exchange 2010 due to the store schema changes and the ESE changes, our IO profile is vastly different. As a result of that Outlook Online Mode and Outlook Cached Mode behave the same from an IO perspective


E2010, online mode and cached mode have the same IO profile.

And also in the Exchange 2010 Technical Library:
Due to the changes in the store schema and Extensible Storage Engine (ESE), Outlook Online Mode clients now generate the same I/O profile as Outlook Cached Exchange Mode Clients.
Pesos Posted - 04/22/2012 : 3:47:23 PM
I knew 2010 was more efficient in the way it sequences I/O but I wasn't aware there was no impact whatsoever to online mode? Really?
Jazzy Posted - 04/22/2012 : 1:20:41 PM
Yeah, I can't remember I've seen an Exchange 2007/2010 environment on physical hardware, everything nowadays is on a hypervisor and with SAN storage.

By the way, for Exchange 2010 it makes no difference in IOPs if your clients work in cached or online mode so don't expect too much of that.
Pesos Posted - 04/22/2012 : 12:12:10 PM
All our exchange servers are now on hyper-v, so the whole disk/lun thing kind of goes out the window to a certain extent. We have a mix of 15k sas for primary databases and 7.2k sata for archive databases. Currently the majority of our 700ish mailboxes are accessed via online mode in citrix sessions, but we are gradually phasing citrix out in favor of local outlook clients in cached mode, which should continue to relieve disk load - makes a big difference.

We have about 500gb total data and try to keep our databases around 20gb or less in size since our datacenters are limited to a 18mbit link (about 75ms latency). These dbs are spread over three separate dags (two exchange organizations).
Jazzy Posted - 04/20/2012 : 1:56:42 PM
Originally posted by clarinathan

Also we tend to go for a mapping of one single database per disk.

Are you really seeing that a lot?
clarinathan Posted - 04/20/2012 : 12:46:00 PM
You're right.... in the sense that there is no right answer.

My feeling is that if you look at a large deployment, say 50,000 - 100,000 users, then you would just have too many databases if you went for small ones!
Also we tend to go for a mapping of one single database per disk. So in that case you would use more smaller disks and have to rack them etc etc.

Agreed though in smaller deployments especially where WAN links are an issue (latency is key), more smaller databases are useful. This allows the replication engine to be more efficient over high latency links because we use a single TCP socket for each DB so more smaller ones, means more overall sockets are used.

Pesos Posted - 04/19/2012 : 01:21:43 AM
Interesting, I tend to find the opposite :-) Reseeds are rare but an inevitability - and it is far easier to reseed one of many smaller dbs than one gigantic one. Then again, you're probably dealing with a lot more data than me, and faster WAN connections!
Jazzy Posted - 04/19/2012 : 12:40:11 AM
Fot me the main reason is that people tend to place every DB on it's own LUN. And becasue you need to have sufficient free space on the volumes, this means a lot more overhead than with a single database on a single LUN. Also you have to monitor disk usage for many LUNs now instead of a single LUN.

Also, sometimes DAG replication fails for some reason and you have to reseed the database. With 12 databases in stead of 1, it's way more likely you run into these kind of issues.

With multiple databases you have to consider an equal load, it makes no sense to have almost empty databases and fast growing ones. Exchange has no native mechanism to do so, this means we end up with unequal load, further increasing the overhead in storage and requiring attention of the administrators to move mailboxes.

All in all I think management is much easier when you have a single database as opposed of 12.

Pesos Posted - 04/18/2012 : 10:55:38 PM
Just to play devil's advocate, what is the problem with having a few smaller databases? I find it to be far more manageable in practice.
clarinathan Posted - 04/18/2012 : 04:17:52 AM
Agreed, that makes much more sense. These days we are looking at databases being significantly bigger. It is often feasible for example to end up with approx. 300 users with 5GB mailboxes on a 2TB disk.

Obviously that does depend on making appropriate use of the replication capabilities to protect against SPoF.

Jazzy Posted - 04/17/2012 : 4:39:19 PM
Did the math, 12 x 80 GB is that you currently have < 1 TB. Could have that in a single DB easy,
Jazzy Posted - 04/17/2012 : 4:26:52 PM
Less databases is better, and 18 is quite a lot indeed. You mentioned the most important factor to consider, the amount of time it takes to perform a restore. Corruption is a factor too but in Exchange 2010 with all its self healing features the risk on corruption is much smaller than it was with 2003. Also because of the multiple copies of the database, a situation where you have to take down a database for maintenance is not very likely to happen.

So with a DAG and multiple copies there's no recommendation for a size limit, the only limit is what the filesystem supports but that's like 16 TB or something. In practice databases of a couple of TB are quite common, especially when you have a fast snapshot based way of restore.

How does that sound, place all users in two or three databases?
MadCow Posted - 04/17/2012 : 4:03:57 PM

Ok ... Currently Ex2007 we have SGs like ... A-B, C-D and so on ...

Users whose names start with A we drop their mailbox in the respective SG A-B and so on. We are thinking to keep the DB size no bigger than 80GB to avoid corruption and have quick restores.

On Ex2010 .. we are thinking to create DBs A-B, C-D, E, F and G-K and so on and thats why we have 12 DBs. Does this make sense or please advise if we can take a better approach? I thought that 12 DBs were too much for us ... have around 1800 mailboxes.

Jazzy Posted - 04/17/2012 : 3:58:41 PM
So why do you have 12 databases? That's quite a lot.
MadCow Posted - 04/17/2012 : 3:50:01 PM

Thanks Jetze.

I think we will have all DBs active on one single server since the servers are pretty powerful in terms of hardware.
Jazzy Posted - 04/17/2012 : 2:33:28 PM
So assuming you want HA for every database, both servers will host 12 databases. Your question is if you want all all databases active on a single server or have 6 active and 6 passive databases on both servers? This depends on how you sizes the servers, can they handle 100% of the users with good performance, then it doesn't really matter. When you sized for 50% of the users and anticipated on a performance penalty when a server fails, you want to have 6 databases active on both servers.
Jazzy Posted - 04/17/2012 : 1:46:13 PM
From a server point of view, all servers are actieve because they can host active or passive copies of databases. A single database can have 16 copies with only one active copy. This is of course very different from Exchange 2003 and 2007.

So, what are your variables? How many users for example and what do you want to achieve with high availability?
MadCow Posted - 04/17/2012 : 1:43:10 PM
Servers with Databases in one site/office.

I am looking at hosting 12 Databases/Server. I have 2 Servers.
Jazzy Posted - 04/17/2012 : 1:22:08 PM
Explain A/A or A/P. Are we talking databases, servers, sites, datacenters?

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