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Mirroring and replication capabilities

Double-Take Availability mirrors and replicates file and directory data stored on any Windows file system (FAT, FAT32, NTFS4, and NTFS5). Mirrored and replicated items also include Macintosh files, compressed files, NTFS attributes and ACLs (access control list), dynamic volumes, files with alternate data streams, sparse files, and encrypted files. Files can be mirrored and replicated across mount points, even though mount points are not created on the target. Some reparse points are mirrored and replicated, including CommVault Data Migrator and BridgeHead Software HT FileStore.

Double-Take Availability does not mirror or replicate items that are not stored on the file system, such as physical volume data and registry based data. Additionally, Double-Take Availability does not mirror or replicate NTFS extended attributes, registry hive files, Windows or any system or driver pagefile, system metadata files ($LogFile, $Mft, $BitMap, $Extend\\$UsnJrnl, $Extend\\$Quota, $Extend\\$ObjId, and $Extend\\$Reparse), hard links, or the Double-Take Availability disk-based queue logs. The only exception to these exclusions is for the full-server workloads. If you are protecting your system state and data using full-server protection, Double-Take Availability will automatically gather and replicate all necessary system state data, including files for the operating system and applications.

Note the following replication caveats.

1. If you have mixed file systems, keep in the mind the following.
1. If, on your source, you have a FAT volume mounted on a directory which resides on an NTFS volume, these files will not be mirrored, regardless of the target file system. Replication will work correctly. To work around this issue, make sure both volumes are NTFS.
2. If you are mirroring/replicating from an NTFS source to a FAT target, you may see additional error messages in your Double-Take Availability log file because the target file system cannot handle the NTFS attributes or file permissions. For example, if your job contains files with alternate data streams, you will see messages indicating that there are unfinished operations because the FAT file system cannot store the alternate data stream information.
3. If you select a compressed file or folder from an NTFS partition and replicate it to a FAT target, the attributes are lost, but the data is maintained.
2. If any directory or file contained in your job specifically denies permission to the system account or the account running the Double-Take service, the attributes of the file on the target will not be updated because of the lack of access. This also includes denying permission to the Everyone group because this group contains the system account.
3. If you select a dynamic volume and you increase the size of the volume, the target must be able to compensate for an increase in the size of the dynamic volume.
4. If you select files with alternate data streams, keep in mind the following.
1. Alternate data streams are not included in the job size calculation. Therefore, you may see the mirror process at 99-100% complete while mirroring continues.
2. The number of files and directories reported to be mirrored will be incorrect. It will be off by the number of alternate streams contained in the files and directories because the alternate streams are not counted. This is a reporting issue only. The streams will be mirrored correctly.
3. Use the checksum option when performing a difference mirror or verification to ensure that all alternate data streams are compared correctly.
4. If your alternate streams are read-only, the times may be flagged as different if you are creating a verification report only. Initiating a remirror with the verification will correct this issue.
5. If you select encrypted files, keep in mind the following.
1. Only the data, not the attributes or security/ownership, is replicated. However, the encryption key is included. This means that only the person who created the encrypted file on the source will have access to it on the target.
2. Only data changes cause replication to occur; changing security/ownership or attributes does not.
3. Replication will not occur until the Windows Cache Manager has released the file. This may take awhile, but replication will occur when Double-Take Availability can access the file.
4. When remirroring, the entire file is transmitted every time, regardless of the remirror settings.
5. Verification cannot check encrypted files because of the encryption. If remirror is selected, the entire encrypted file will be remirrored to the target. Independent of the remirror option, all encrypted files will be identified in the verification log.
6. Empty encrypted files will be mirrored to the target, but if you copy or create an empty encrypted file within the job after mirroring is complete, the empty file will not be created on the target. As data is added to the empty file on the source, it will then be replicated to the target.
7. When you are replicating encrypted files, a temporary file is created on both the source and target servers. The temporary file is automatically created in the same directory as the Double-Take Availability disk queues. If there is not enough room to create the temporary file, an out of disk space message will be logged. This message may be misleading and indicate that the drive where the encrypted file is located is out of space, when it actually may be the location where the temporary file is trying to be created that is out of disk space.
6. If you are using mount points, keep in mind the following.
1. By default, the mount point data will be stored in a directory on the target. You can create a mount point on the target to store the data or maintain the replicated data in a directory. If you use a directory, it must be able to handle the amount of data contained in the mount point.
2. Recursive mount points are not supported. If you select data stored on a recursive mount point, mirroring will never finish.
7. Double-Take Availability supports transactional NTFS (TxF) write operations, with the exception of TxF SavePoints (intermediate rollback points).

1. With transactional NTFS and Double-Take Availability mirroring, data that is in a pending transaction is in what is called a transacted view. If the pending transaction is committed, it is written to disk. If the pending transaction is aborted (rolled back), it is not written to disk.

   During a Double-Take Availability mirror, the transacted view of the data on the source is used. This means the data on the target will be the same as the transacted view of the data on the source. If there are pending transactions, the Double-Take Availability Target Data State will indicate Transactions Pending. As the pending transactions are committed or aborted, Double-Take Availability mirrors any necessary changes to the target. Once all pending transactions are completed, the Target Data State will update to OK.

   If you see the pending transactions state, you can check the Double-Take Availability log file for a list of files with pending transactions. As transactions are committed or aborted, the list is updated until all transactions are complete, and the Target Data State is OK.

2. During replication, transactional operations will be processed on the target identically as they are on the source. If a transaction is committed on the source, it will be committed on the target. If a transaction is aborted on the source, it will be aborted on the target.
3. When failover occurs any pending transactions on the target will be aborted before the source identity is assigned to the target.
4. Double-Take Availability restore functions as a mirror, except the roles of the source and target are reversed. The transacted view of the data on the target is restored to the source. As pending transactions are committed or aborted on the target, Double-Take Availability restores any necessary changes to the source. Once all pending transactions are completed, the restoration is complete and replication will continue from the target to the source.
5. If you have restored your data before starting the failback process, make sure the restoration process does not have pending transactions and is complete before starting failback. If you are restoring your data after the failback the process has completed, users will not be accessing the data once failback occurs, so there are no opportunities for pending transactions.
8. Double-Take Availability supports Windows 2008 symbolic links and junction points. A symbolic link is a link (pointer) to a file. Junction points are also links, but to folders and volumes.
1. If the link and the file/folder/volume are both in your job, both the link and the file/folder/volume are mirrored and replicated to the target.
2. If the link is in the job, but the file/folder/volume it points to is not, only the link is mirrored and replicated to the target. The file/folder/volume that the link points to is not mirrored or replicated to the target. A message is logged to the Double-Take Availability log identifying this situation.
3. If the file/folder/volume is in the job, but the link pointing to it is not, only the file/folder/volume is mirrored and replicated to the target. The link pointing to the file/folder/volume is not mirrored or replicated to the target.
4. Junction points will not be processed for orphan files.
9. Short file names are not supported on FAT file systems.
10. If you have the Windows NtfsDisable8dot3NameCreation setting enabled (set to 1) on the source but disabled (set to 0) on the target, there is a potential that you could overwrite and lose data on the target because of the difference in how long file names will be associated with short files names on the two servers. This is only an issue if there are like named files in the same directory (for example, longfilename.doc and longfi~1.doc in the same directory). To avoid the potential for any data loss, the NtfsDisable8dot3NameCreation setting should be the same on both the source and target.
11. Double-Take Availability can replicate paths up to 32,760 characters, although each individual component (file or directory name) is limited to 259 characters. Paths longer than 32760 characters will be skipped and logged.
12. If you rename the root folder of a job, Double-Take Availability interprets this operation as a move from inside the job to outside the job. Therefore, since all of the files under that directory have been moved outside the job and are no longer a part of the job, those files will be deleted from the target replica copy. This, in essence, will delete all of your replicated data on the target. If you have to rename the root directory of your job, make sure that the job is not connected.
13. Keep in mind the following caveats when including and excluding date for replication.
1. Do not exclude Microsoft Office temporary files from your job. When a user opens a Microsoft Office file, a temporary copy of the file is opened. When the user closes the file, the temporary file is renamed to the original file and the original file is deleted. Double-Take Availability needs to replicate both the rename and the delete. If you have excluded the temporary files from your job, the rename operation will not be replicated, but the delete operation will be replicated. Therefore, you will have missing files on your target.
2. When Microsoft SQL Server databases are being replicated, you should always include the tempdb files, unless you can determine that they are not being used by any application. Some applications, such as PeopleSoft and BizTalk, write data to the tempdb file. You can, most likely, exclude temporary databases for other database applications, but you should consult the product documentation or other support resources before doing so.
3. Some applications create temporary files that are used to store information that may not be necessary to replicate. If user profiles and home directories are stored on a server and replicated, this could result in a significant amount of unnecessary data replication on large file servers. Additionally, the \Local Settings\Temporary Internet Files directory can easily reach a few thousand files and dozens of megabytes. When this is multiplied by a hundred users it can quickly add up to several gigabytes of data that do not need to be replicated.
4. Creating jobs that only contain one file may cause unexpected results. If you need to replicate just one file, add a second file to the job to ensure the data is replicated to the correct location. (The second file can be a zero byte file if desired.)
14. Double-Take Availability does not replicate the last access time if it is the only thing that has changed. Therefore, if you are performing incremental or differential backups on your target machine, you need to make sure that your backup software is using an appropriate flag to identify what files have been updated since the last backup. You may want to use the last modified date on the file rather than the date of the last backup.
15. Keep in mind the following caveats when using anti-virus protection.
1. Virus protection software on the target should not scan replicated data. If the data is protected on the source, operations that clean, delete, or quarantine infected files will be replicated to the target by Double-Take Availability. If the replicated data on the target must be scanned for viruses, configure the virus protection software on both the source and target to delete or quarantine infected files to a different directory that is not in the job. If the virus software denies access to the file because it is infected, Double-Take Availability will continually attempt to commit operations to that file until it is successful, and will not commit any other data until it can write to that file.
2. You may want to set anti-virus exclusions on your source to improve replication performance. There are risks associated with making exclusions, so implement them carefully. For more information, see the Microsoft article 822158 Virus scanning recommendations for Enterprise computers that are running currently supported versions of Windows.
16. SQL Server 2005 or later may not initialize empty space when the database size increases due to the auto grow feature. Therefore, there is nothing for Double-Take Availability to replicate when this empty space is created. When the empty space is populated with data, the data is replicated to the target. A verification report will report unsynchronized bytes between the source and target due to the empty space. Since the space is empty, the data on the source and target is identical. In the event of a failure, the SQL database will start without errors on the target.
17. If you are running Symantec version 10 or later, you may receive Event message 16395 indicating that Double-Take Availability has detected a hard link. Symantec uses a hard link to recover from a virus or spyware attack. Double-Take Availability does not support hard links, therefore, the Event message is generated, but can be disregarded.  
18. If you have reparse points in your data set, do not include the HSM data in the data set if the HSM driver is not loaded.Additionally, if you are using an HSM solution, once your source has failed over to the target, do not archive any files. Archiving files after a failover could cause corruption.
19. If you are using Windows 2008 and the Microsoft Windows Update feature, schedule your updates outside the times when a mirroring operation (initial mirror or remirror) is running. Windows updates that occur during a mirror may cause data integrity issues on the target.