Lesson 2: File Replication Service | MCSE Training Kit(c) Microsoft Windows 2000 Accelerated 2000

FRS is the file replication service in Windows 2000 Server. It is used to copy and maintain files on multiple servers simultaneously and to replicate the Windows 2000 System volume (SYSVOL) on all domain controllers. In addition it can be configured to replicate data for domain Dfs roots.

After this lesson, you will be able to

Describe what data can be replicated by FRS
Configure replication for domain Dfs roots
Describe the replication process in the Active Directory services and FRS

Estimated lesson time: 25 minutes

FRS Replication

FRS is installed automatically on all Windows 2000 Servers. It is configured to start automatically on all domain controllers and manually on all standalone and member servers. Although Active Directory replication and the FRS are independent of each other, they share common replication topology, terminology, and methodology. In fact, the Active Directory store uses FRS to synchronize the directory among all domain controllers.

Each Windows 2000 domain has one or more servers that serve as domain controllers. Each domain controller stores a complete copy of Active Directory store for its domain and is involved in managing changes and updates to the directory.

Within a site, Active Directory service automatically generates a ring topology for replication among domain controllers in the same domain. The topology defines the path for directory updates to flow from one domain controller to another until all domain controllers receive the directory updates.

The ring structure ensures that there are at least two replication paths from one domain controller to another; if one domain controller is down temporarily, replication still continues to all other domain controllers.

Active Directory service uses multimaster replication, in which no one domain controller is the master; instead, all domain controllers within a domain are equivalent.

Active Directory service periodically analyzes the replication topology within a site to ensure that it is still efficient. If you add or remove a domain controller from the network or a site, Active Directory service reconfigures the topology to reflect the change.

Sites and Replication

A site is made up of one or more IP subnets that identify a group of well-connected computers. Only those subnets that share fast and reliable network connections of at least 512 kilobits per second (Kbps) should be combined.

Domain structure and site structure are maintained separately in Active Directory services. A single domain can include multiple sites, and a single site can include multiple domains or parts of multiple domains, as shown in Figure 24.5.

Figure 24.5 A single domain with a single site, a single domain with multiple sites, and multiple sites with multiple domains

There are two types of replication: intra-site replication and inter-site replication.

Intra-Site Replication

Intra-site replication has the following characteristics:

It occurs between domain controllers within a site.
Replicated data is not compressed.
The default replication interval is 5 minutes.
Replication is trigger-based, that is, a notification is sent when certain criteria are met, and then the data is pulled from the server.

Inter-Site Replication

Inter-site replication has the following characteristics:

It occurs between domain controllers in different sites.
You can specify the time when inter-site replication should occur. The default replication interval is 3 hours.
You can specify the network transport used.
It is compressed, regardless of the transport used.
Compression reduces the data on the network by 88 to 90 percent.

One disadvantage of inter-site replication is that it is not configured automatically; it must be configured by an administrator.

Knowledge Consistency Checker

Within a site, a process called the Knowledge Consistency Checker (KCC) generates a ring topology for replication among domain controllers in the same domain. The generated topology defines paths for directory updates to flow from one domain controller to another until all domain controllers have received the directory updates.

This ring structure guarantees that there are at least two replication paths from one domain controller to another, ensuring that if one domain controller is temporarily down, replication continues to all other domain controllers. In addition, the ring structure is created such that an update takes, at most, three "hops" from the domain controller where it originates to any other domain controller in the site.

The KCC periodically analyzes the replication topology within a site to ensure that the replication topology is efficient. If a domain controller is added or removed from the network or a site, the KCC reconfigures the topology to reflect the change.

NOTE
Administrators can make modifications to the replication topology, including changing the schedule for inter-site replication, to meet the requirements of an organization.

Unique Sequence Numbers

When a directory object is updated at a domain controller, either through a change that a user or administrator makes or by replication from another domain controller, the domain controller assigns the change a unique sequence number (USN). Each domain controller maintains its own USNs and applies USNs incrementally to each directory change made at the domain controller.

When the domain controller writes the change into the directory, it also writes the USN of the change with the property.

Each domain controller maintains a table of the USNs that it receives from every other domain controller in the domain, and the table lists the highest USN that is received from each domain controller. Each domain controller then periodically notifies the other domain controllers in the domain that it has received changes and sends its current USN. Each domain controller that receives this message checks its USN table for the last USN that it received from the sending domain controller. If there are changes and the domain controller has not received them, it requests that only the changes be sent.

Using USNs eliminates the need for precise timestamps for changes and for time to be synchronized precisely among domain controllers within a domain. However, timestamps are still applied to directory changes for tie breaking.

Using USNs also simplifies recovery after a failure. When a domain controller is running again after a failure, it restarts replication by asking each of the other domain controllers for changed USNs greater than the last USN in the table for that domain controller. Because the table is updated automatically as the change is applied, interrupted replication cycles pick up exactly where they left off, with no loss or duplication of updates.

Implementing FRS

Implementation of FRS consists of several phases: replicating SYSVOL, replicating domain Dfs roots, and configuring FRS for inter-site replication.

Replicating SYSVOL

Changes to the %systemroot%\SYSVOL directory on any domain controller are automatically replicated to the other domain controllers within the site. The replication topology and process is separate from but identical to Active Directory replication. When an administrator adds, removes, or modifies the contents of %systemroot%\SYSVOL folder on any domain controller, those changes are replicated to the other domain controllers within the site automatically.

The default folder structure is as follows:

%systemroot%\SYSVOL\Sysvol\domain_name\Policies
%systemroot%\SYSVOL\Sysvol\domain_name\Scripts

Any files and folders added to %systemroot%\SYSVOL\Sysvol\domain_name are automatically replicated.

Replicating Dfs Fault-tolerant Roots

Dfs uses FRS to replicate data in domain Dfs links. When changes are made to a domain Dfs link that is part of a domain Dfs root, the changes are automatically replicated to other replica members.

Dfs and file replication support the following features:

Multimaster replication replicates modified files and modified ACLs when a file is closed.
Files can be modified on any replica member.
Only Windows 2000 NTFS volumes have the potential to replicate. Other shares can be published as alternates, but no replication occurs.
Replication is journal based.
Replication is Remote Procedure Call (RPC) based.
FRS topology follows Active Directory replication topology.

The process of Dfs replication consists of the following steps:

A file changes. This is noted when a user closes a file.
NTFS makes an entry in the NTFS Change Log.
FRS monitors the NTFS journal for changes to Dfs links.
FRS makes an entry into its own journal.
FRS generates a staging file of the file change.
FRS holds on to changes until scheduled to replicate.
The destination pulls the staging file and applies the new files.

Adding Replica Dfs Root Servers

Each Dfs root or link can reference a replicated set of shared resources. Dfs clients automatically select the nearest replica based on site topology information.

To add Dfs replica servers to a Dfs domain root or link, right-click the Dfs root in the Distributed File System Manager tool, click New, and then click Root Replica. Enter the UNC path for the replica server and share.

Enabling Dfs Replication

Dfs replication is disabled by default. To enable replication, right-click the Dfs root or Dfs link in the Distributed File System snap-in, and then select Replication Policy. Highlight every server in the replica set that you want to participate in FRS replication, and click the Enable button. Servers that do not participate in replication must be synchronized manually.

Configuring FRS for Inter-Site Replication

You can configure inter-site replication by using the Active Directory Sites and Services snap-in. To configure the FRS settings, you must create a new site link for the inter-site transport protocol listed in the console tree. Once you've created the site link, right-click the site link object and click Properties. The Properties dialog box opens. You can now configure the inter-site replication as necessary.

Lesson Summary

FRS is the automatic file replication service in Windows 2000 Server. It copies and maintains files on multiple servers. There are two types of replication: intra-site replication and inter-site replication. Sites are defined as one or more subnets that identify a group of well-connected computers. Within a site, the KCC process automatically generates a ring topology for replication among domain controllers in the same domain. Implementing FRS consists of several phases, including replicating SYSVOL, replicating domain Dfs roots, and configuring FRS.