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In addition to the core Double-Take Availability requirements, use these requirements for full-server protection.
Microsoft .NET Framework—The source and target servers require the Microsoft .NET Framework version 3.5 Service Pack 1. This version is not included in the .NET version 4.0 release. Therefore, even if you have .NET version 4.0 installed, you will also need version 3.5.1. You can install this version from the Double-Take Availability CD, via a web connection during the Double-Take Availability installation, or from a copy you have obtained manually from the Microsoft web site.
If you are installing on Server Core 2008 R2, .NET 3.5.1 functionality is included with the operating system but is disabled by default. You will need to enable it prior to installing Double-Take Availability. See your Server Core documentation for the specific commands to enable the .NET functionality.
Operating system version
The source and target must have the same operating system. For example, you cannot have Windows 2003 on the source and Windows 2008 on the target. The two servers do not have to have the same level of service pack or hotfix. Windows 2003 and 2003 R2 are considered the same operating system, however the Windows 2008 and 2008 R2 releases are considered different operating systems. Therefore, you can have Windows 2003 on the source and Windows 2003 R2 on the target, but you cannot have Windows 2008 on the source and Windows 2008 R2 on the target. The Windows edition (Standard, Enterprise, and so on) does not have to be the same.
The target cannot be a domain controller. Ideally, the target should not host any functionality (file server, application server, and so on) because the functionality will be removed when failover occurs.
If your source is a domain controller, it will start in a non-authoritative restore mode after failover. This means that if the source was communicating with other domain controllers before failover, it will require one of those domain controllers to be reachable after failover so it can request updates. If this communication is not available, the domain controller will not function after failover. If the source is the only domain controller, this is not an issue.
Additionally, if your source is a domain controller, you will not be able to reverse protection.
Architecture
The source and the target must have the same architecture. For example, you cannot failover a 32-bit server to a 64-bit server.
Processors
There are no limits on the number or speed of the processors, but the source and the target should have at least the same number of processors. If the target has fewer processors or slower speeds than the source, there will be performance impacts for the users after failover.
Memory
The target memory should be within 25% (plus or minus) of the source. If the target has much less memory than the source, there will be performance impacts for the users after failover.
Network adapters
You must map at least one NIC from the source to one NIC on the target. If the source has more NICs than the target, some of the source NICs will not be mapped to the target. Therefore, the IP addresses associated with those NICs will not be available after failover. If there are more NICs on the target than the source, the additional NICs will still be available after failover.
File system format
The source and the target must have the same file system format. For example, an NTFS volume cannot be sent to a FAT volume.
HAL type and version
The Windows hardware abstraction layer (HAL) refers to a layer of software that deals directly with your computer hardware. The HAL type and version do not have to be identical, but they must be compatible between the source and the target. If the two are incompatible, Double-Take Availability will warn you. In that case, you must upgrade or downgrade the target.
System volume
The target must have the same system volume as the source. The system volume is the disk volume that contains the hardware-specific files that are needed to start Windows. The system volume might be the same volume as the boot volume, but that configuration is not required.
Logical volumes
There are no limits to the number of logical volumes, although you are bound by operating system limits. For each volume you are protecting on the source, the target must have a matching volume. For example, if you are protecting drives C: and D: on the source, the target cannot have drives D: and E:. In this case, the target must also have drives C: and D:.
System path
The source and the target must have the same system path. The system path includes the location of the Windows files, Program Files, and Documents and Settings.
Double-Take Availability path
Double-Take Availability must be installed on the same volume on the source and the target.
The target must have enough space to store the data from the source. This amount of disk space will depend on the applications and data files you are protecting. The more data you are protecting, the more disk space you will need.
The target must also have enough space to store, process, and apply the source's system state data. If you will be enabling reverse protection, the source must have enough space to store, process, and apply the target's system state data. In either case, the size of the system state will depend on the operating system and architecture. Windows 2003 operating systems need at a minimum 2-3 GB of free space for the system state. Windows 2008 operating systems need at a minimum 7-9 GB of free space for the system state. Windows 2008 R2 operating systems need 10 GB of free space for the system state. These minimums are for a clean operating system installation. Operating system customizations will increase the disk space requirement.