3.5 Scaling the Active Directory 165
Chapter 3
Local quorum: you just need to have a locally attached disk where
quorum information is stored, and that must be present. Local quo-
rum is appropriate for any environment that does not have a storage
interconnect. Local quorum is appropriate for lone-wolf clusters—in
other words, single-node clusters.
Majority node set: you need to have a majority of the nodes up and
running the Microsoft cluster service for the cluster to be considered
healthy and operable. This mode of quorum is only applicable for
clusters that have three nodes or more and are appropriate in that you
do not need to provision storage resources access to each of the nodes
of the cluster. This method is particularly interesting for geographi-
cally dispersed clusters.
iSCSI-Based Cluster
This is the newbie in the list of enhancements of the clustering software in
Windows Server 2003, which appeared with SP1 of Windows Server 2003.
When using the Microsoft iSCSI Software Initiator, you can now build up
to eight-node clusters. This is a significant improvement because as data
networks evolved, iSCSI is becoming more and more a viable way of con-
necting a Windows Server to back-end storage networks in block-mode
across a simple IP network. Of course, with the I/O workload that is typi-
cally demanded from Microsoft Exchange, you might hesitate to deploy
storage networks over iSCSI rather than a fibre channel, which has proven
itself in terms of high-performance transport.
SAN Boot Support
Although it was possible to build diskless servers in Windows 2000 clusters,
the requirement was to use a separate fibre channel path to the boot units
from the shared storage disk units. This required users to use 4 (2 × 2) HBA
(Host Bus Adapter) for ensuring a redundant path to the data disks (first
pair of HBA) and to the system disks (second pair). With Windows Server
2003 and the combined use of STORport drives, it is now possible to build
clusters with diskless servers (i.e., servers that boot off the storage area net-
work) and use the same data path for the system disks and the data disks in
the cluster.
3.5 Scaling the Active Directory
There are two ways in which the Active Directory can scale: using a scale-up
model, where you attempt to store a large number of objects in the Active
166 3.5 Scaling the Active Directory
Directory, and using a scale-out model, where you try to deploy a large
number of Active Directory domain controllers and servers. A combination
of the two can be used by corporations that decide to have a global roll-out
of the Active Directory, which results in gathering a large number of objects
into a single forest (in separate domains, however) and a large number of
domain controllers due to the presence of multiple sites.
Scale-Up the Active Directory
The scale-up of the Windows Active Directory service is typically achieved
across two dimensions:
Growing the number of objects in the Windows Active Directory;
Diminishing the number of domain controllers and GC servers in
the infrastructure: a single Windows Active Directory server would
serve more client machines/Microsoft Exchange servers.
When I first worked on this book, we looked at growing the number of
items in the Windows Active Directory by loading it with the U.S. phone
book. It was a simple experience that consisted of adding as many contacts
as possible and making observations on the scalability factor of the Win-
dows Active Directory service. That corresponded to approximately 100
million objects stored into the directory. We found interesting things as the
result of this experience:
1. The size of the database would grow in a linear fashion. That
was of little surprise to us. The Active Directory database engine
is a variant of the ESE (discussed in Chapter 1 of this book),
which has proved in past versions of Exchange to scale quite well
in capacity. The only issue with it is the single-file database
model, but for most deployments that shouldnt provide too
much of a problem.
2. The response time for a transaction would not grow with the size
of the database. This is a very important aspect of the scalability
of the Active Directory, and in general of any directory service,
based on solid database technology. With 100 million objects, we
found that for a given request, more time was spent in rendering
the information for the user’s browser than actually looking up

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