What is the difference between synchronization and replication?
When first deployed, it is necessary for Double-Take® to create an initial copy of the data from the source onto the target. This is necessary to then allow replication to update it with just the changes. Synchronization is sometimes referred to as 'mirroring', where a mirror copy of the entire data set is created. The only other time it is necessary to resynchronize is after the connection between the source and target is broken, either due to a server shutdown, network disconnect, etc. Should this occur, Double-Take will perform a resynchronization once the connection is reestablished. Resynchronization doesn't copy all of the source data, it copies just the differences between the source and target. Resynchronization has a few options to determine what is different on the target. It can either check file attributes such as size, modification date, etc. and copy differences based on this information or it can do a block-level checksum compare. This is configurable within the user interface.

Does Double-Take replicate the entire file when it is changed?
No. Unlike other replication products that replicate entire files or whole blocks when only small changes are made, Double-Take replicates just the bytes that are changed. This allows Double Take to provide efficient use of network resources and minimize latency between the source and target servers.

Are changes applied to the target server immediately?
Double-Take is a driver-level application that captures changes at the file-system level as they are made on the source server. If configured appropriately, and the network bandwidth is available, these changes will be immediately replicated to the target server(s). Most often this is the case, however there are times when your business may require changes to be transmitted at a lower priority than other tasks currently occurring. In this case, Double-Take offers numerous configuration options that allow transmissions to be controlled to best suit your needs.

Can the replicated data on the target be accessed by users while replication is enabled?
Yes. Double-Take does not by default lock the data on the target server. This allows read and write access on the target data. This is very useful for data distribution to remote branch offices, testing, etc.

Does Double-Take support Unicode?
Yes. Double-Take supports all Unicode files for replication, including non-English alphabet characters (Japanese, etc.). The Double-Take Management Console also supports Unicode characters for configuring replication sets, etc.

Are ACL's replicated and maintained?
Yes. All ACL's are copied to the target replica during the initial synchronization and are also maintained during replication. Unlike other replication products, if an ACL is changed on a source file, this change WILL be replicated to the target. This is critical in maintaining file and data security in your environment.

Are shares replicated and maintained?
Yes. Any source file or folder that has a share defined for it will have it's share properties replicated to the target during initial synchronization. Any changes or additions to shares will be replicated to the target as well during real-time replication.

Will Double-Take replicate encrypted files?
Yes. All encrypted files will be copied during the initial synchronization and will continue to be replicated during the real-time replication process. Encryption remains enabled on the target replica to ensure proper security is maintained.

Can Double-Take run a synchronization and replication simultaneously?
Yes. During a synchronization, Double-Take can also perform real-time replication. This ensures that any changes taking place during the synchronization process are also captured and replicated. Once the synchronization is complete the target replica is an exact copy of the source at that point.

Will Double-Take work in a Microsoft® Cluster environment?
Yes. Double-Take is able to replicate data from clustered servers to another cluster or to a standalone server. Double-Take also allows for failover from the cluster to the target server, even if the target is not a cluster. This greatly enhances the protection and availability of the cluster. For an even greater level of availability and protection of your MSCS environment, NSI offers GeoCluster®. Click here for more information on GeoCluster.

Can Double-Take be controlled as to the amount of bandwidth it utilizes?
Yes. Double-Take has numerous settings to allow customized control of how and when it runs and how much bandwidth it can use.

Do I need to reboot my servers when installing Double-Take?
Yes. You will be required to reboot your servers just once during the initial installation of Double-Take. Once running, Double-Take requires no additional reboots of the server for any reason, including new files, applications, write-locks, start/stop of applications, etc.

Where can I find a list of error codes?
The Double-Take Users Guide which can be found on the installation CD contains a list and explanation of error codes.

What applications does Double-Take support?
Because Double-Take works at the file-system level, any application that utilizes the NTFS file system is fully supported by Double-Take. Whether it be Microsoft Exchange, SQL Server™, Oracle®, or any other third-party or home-grown application, Double-Take will ensure that it's data is accurately and efficiently replicated to a remote server.

Will Double-Take slow down my production applications?
While Double-Take does require some CPU cycles to run, it is typical that it will consume an average of 5%, sometimes more when necessary and when available, and sometimes less. This may vary depending on your particular environment, size of your systems, amount of data being changed, etc. However unlike synchronous replication solutions, Double-Take does not prevent the applications from processing data while Double-Take replicates it. Double-Take will capture the data and write it to cache until it can be transmitted to the target, however the applications will continue to process and write data.

How does Double-Take compare to snapshot products?
While snapshots are useful in providing for rapid recovery of files from different points in time, they typically do not allow for creation on a separate disk or server and are not full data images, they typically capture just the changed data. As such they are not able to provide recovery in the event a server or disk were to fail or become unavailable. Double-Take on the other hand provides local and remote replication and creates a complete image of the production data that can be used for rebuilding, restoring, and failover.

Is Double-Take synchronous or asynchronous?
Double-Take provides asynchronous replication. Although your particular environment will yield different results, Double-Take provides very fast and efficient replication of data resulting in a very small latency between the source and target data. This also allows Double-Take to run without impacting production applications, as synchronous solutions require that the target data be written before the application can continue.

What is the difference between hardware-based and SAN-based replication solutions?
Array-based solutions are much more expensive than Double-Take and are not heterogeneous, meaning that they support only that one type and/or vendors hardware. Configuration is often more difficult as well requiring a greater learning curve and longer time to deployment. With Double-Take, you can replicate any hardware to any hardware, between vendors, SAN, LAN, NAS, and even between Windows® OS. This allows for much greater flexibility in making new purchases, when you make them, and the vendor and products you select. With it's Windows-based GUI, deployment and configuration are simplified resulting in a smaller and less expensive learning curve and a quicker time to deployment. Some SAN-based solutions require additional hardware be purchased and implemented into your SAN, and typically require that at least two of these devices be installed; one at each end of the replication path. These solutions add complexity to your SAN, require additional learning time, and have the potential to impact the overall performance of the entire SAN.

Does Double-Take replace my existing tape backup systems?
No. Tape backup is still required for archival copies of data in case you need to recover data from past dates. Double-Take augments tape backup by providing a real-time online version of data that can be used for immediate recovery or failover, should an outage occur. Double-Take can also greatly enhance and simplify your current backup strategy by allowing backups to be moved from the production servers to offline servers by backing up a replica of the data. This allows your application servers to dedicate all resources on the applications and not on backup. For branch office support, instead of providing backup hardware and software to each remote location, you can centralize all backup operations by utilizing Double-Take to copy the remote locations data to your main data center where backups can be processed by trained personnel and archived along with all other backup media.

Can the data updates/transmissions be encrypted? What is the impact on performance? Can a hardware-assist encryption engine be used?
Because Double-Take replication uses native "server-to-server" IP communication, customers have multiple options for encrypting the data when deemed necessary:
1) Hardware-level encryption from the Routers or other Infrastructure appliance is usually the best solution. Many of our customers use the Internet to connect between their company sites by leveraging VPN's which provide encrypted tunneling. This provides a secure path between sites. One of the empowering features of Double-Take for managing this encryption is that Double Take uses defined ports (default 1100 and 1105). This allows customers simply define those ports as "to be encrypted", as opposed to "everything" on the corporate WAN.
2) In those cases where a customer wants encryption within a given site, we recommend that they use the native "server-to-server" encryption capabilities of Windows 2000 & 2003. Those O/S can define a policy such that all communication from one IP address/range to another is automatically IPSEC/L2TP encrypted - while the "server to workstation" communication isn't.
Third-party Hardware Adapters (e.g. TOE cards) can also be used, as long as they are "under" the networking layer of the server.
In short, Double-Take hands all network communication to the Windows O/S. Any technology that a customer wishes to employ from the network layer down will be transparent to Double Take.

Does Double-Take support open file backups?
Yes. It provides two levels of real-time, open-file backup. First, it supports real-time transaction- based replication of open files to one or more disks or servers located anywhere on a network. Users do not have to be taken offline or close files in order for the files to be replicated by Double-Take. Second, it allows third-party tape systems (installed on the secondary server) to concurrently backup the replicated data to tape media while files remain open - allowing users to implement a real-time, fault tolerant server and tape-based backup system.

What type of system requirements do I need to configure Double-Take? Do I need identical hardware and software for the secondary backup server?
Because Double-Take is a software-based replication system, it does not require identical hardware or dedicated server links. Working with another disk or server located on a LAN or WAN, Double-Take only requires that the secondary server(s) have adequate disk capacity to hold the data being replicated from the primary server. This flexibility provides for solutions such as migrations from one storage media to another, from DAS to a NAS or SAN, from one OS to an upgraded OS, etc.

Can I setup Double-Take to replicate to multiple servers and vice-versa?
Yes. Double-Take can be configured to replicate to one or more servers and can also replicate from many servers to a single server. One-to-many replication enables additional data protection and allows for data distribution; the ability to push production data to remote locations. Many-to-one replication allows for the consolidation of many servers replicated data onto a single server. This is very useful when doing centralized or serverless backups.

Does Double-Take support WAN based backups?
Yes. Double-Take supports WAN-based replication as well as LAN. As such, Double-Take can be used to replicated data between cities, states, and even countries.

What is the difference between mirroring/duplexing and Double-Take? Can I implement both configurations?
Typical disk mirroring, or duplexing, includes one server or storage array with two identical hard disks; each disk is a mirror image of the other. In this situation, the actual mirroring occurs on a block or track basis - at the disk level. As a result, only whole partitions can be mirrored or duplexed. Mirroring usually requires more data to be transmitted as every change may require an entire disk block to be re-mirrored. With Double-Take, files, directories, and server volumes can be selected for replication to any other Windows server and disk/volume; no need for identical hardware. As Double-Take replicates at the byte-level, transferring only the actual bytes that change, network utilization and impact is minimized as is latency between fully synchronized servers.

If my primary server fails, can users log in to the backup server?
Yes. If the primary server fails, users can easily login to the secondary server and continue operations with minimal downtime and data loss. In fact, Double-Take can either automatically or manually have users redirected to the target server the instance the production server fails. Users do not need to make any changes and are likely to not even know there was a failure.

Does Double-Take support failover?
Yes. The failover module runs on the target server and continuously monitors the source servers. Whenever a source server fails, the failover module tells the target server to pick up the failed server's name, shares and IP address. It can also run a failover script, allowing you to automatically restart key applications on the target server.

Do I need to install software on each server I want to protect?
Yes. Double-Take requires that a small and inconspicuous set of files be installed on each server acting as a source or a target.

Does Windows 2000 or Server 2003 include replication functionality like Double-Take?
No. Windows 2000 and Server 2003 include technologies that can be confused with Double Take. Intellimirror - Given the name "mirror", some people assume it is doing data replication. Intellimirror actually is designed to provide "roaming profile" functionality to Windows 2000 clients so their individual preferences are "mirrored" wherever they log in. File Replication Services (FRS) - File Replication services is similar to the "Directory Replicator" included in Windows NT 4.0. Unlike Double-Take, FRS copies entire files every time there is any change (instead of replicating only byte-level changes) and performs periodic copying of closed files. FRS can not replicate files that are open like Double-Take. FRS requires active directory to operate. Microsoft Cluster Service - since it can provide application failover, MSCS is often compared to Double-Take but they really solve different problems and in different ways. For more information on MSCS and its strengths and limitations (and ways to solve them), visit the NSI web site for information on GeoCluster® Volume Shadow Copy Service (VSS) - VSS is part of Windows Server 2003 and Windows Storage Server™ 2003 and provides a way to create local snapshots, or point-in-time copies, of directories and volumes. These are not real time, but rather scheduled, and allow users to recover files from some time in the past. If the drive or server fails, all snapshots are lost. VSS snapshots can not be used to rebuild a failed drive. Double-Take and VSS actually provide very powerful solutions when used in conjunction with each other. For more information on these solutions, please see the Double-Take and VSS whitepaper on the NSI web site.

Why does Double-Take provide compression as an option and not just have it always on?
While compression of transmitted data can have a very positive impact on the data protection solution, it is not always optimal to utilize it, for a few different reasons. Compression requires additional processing which results in higher CPU usage. While some servers and applications can allow for this, there may be some where this is not acceptable. For this reason, NSI chose to allow its customers to determine when and where compression is suitable for their business and their data. Double-Take not only allows users to configure compression for on or off, but also allows users to set compression to one of three different levels, depending on the particular data being replicated. As the leader in enterprise data protection solutions, NSI Software provides the flexibility to meet the varying needs of its customers by allowing them to choose the level that is right for them.

What methods of notification and alerting are available within Double-Take?
Double-Take provides numerous methods of notification for events occurring in the replication and data protection environment. Events and messages can be viewed via the Windows Event Viewer as standard Windows events; can be viewed via a framework or other SNMP-compatible source via our SNMP traps, or via standard email, which could include pagers and cell phones enabled for SMTP. Separate notification methods and recipients can be configured on each dataset being replicated within Double Take providing for different service levels for each dataset.

Can Double-Take restrict administrators to only their set of servers?
While Double-Take does not prevent administrators from viewing or managing certain servers, it does allow individual administrators to create custom views of just a particular set of servers. Instead of searching through tens or hundreds of servers in the Double-Take management console looking for a particular one or few, administrators can create a view that will display just the server(s) that they are concerned with. Scalability is not an issue with Double-Take as hundreds of servers can be viewed in smaller subsets for separation of tasks between administrators and for ease of management.

What functionality is in the 'Replication SSE' product?
The Replication product is a full-functioning Double-Take data protection product - with the exception of failover capabilities. All other NSI features such as in-band control, bandwidth throttling, data verification, advanced-scripting tools, etc. are still included.
The Replication product will not allow failover from or to itself; this restricts it from being a failover target or a failover source. Replication SSE solutions are well suited for solutions like "Data Distribution", "Data Migration", and "Centralized Backup" of remote offices.

What if a customer initially chooses the Replication product but later determines they need failover?
There is an upgrade product (DT4RO2SSE) that can be sold to customers who already have purchased the Replication SSE product. The cost of this upgrade is $695. Combining the upgrade cost with the $1195 for the Replication product results in a total cost that is slightly higher than the full SSE product SKU. If a customer has any thoughts of eventually adding failover, this gives cost-justification to purchase the full SSE product initially and not upgrade at a later date.

Are the SSE products compatible with the other Double-Take products?
Yes, the SSE products are compatible and can coexist with any other product in the Double-Take line. They can all be managed via the same interface as well as replicate to/from each other. The only exception is the inability to fail-over to/from a machine running the Replication SSE product.