Why SnapManager uses NTFS hard links for transaction log archiving

By creating NTFS hard links, SnapManager archives transaction log files without physically copying the log file. During a backup operation, SnapManager archives transaction log files on the live file system to the SnapInfo directory, from which the files later can be retrieved, during a restore operation.

When SnapManager uses a file copy operation to copy the transaction log files from the live file system to the SnapInfo directory, the following resources are consumed:

Management of path names and the copy operation incurs file system overhead and storage system overhead, resulting in slower host and storage system performance.
Each additional copy of a transaction log file not only consumes storage system disk space but also increases the use of overwrite reserve for the LUN.

SnapManager 5.0 and later supports an optimization feature for cases in which the backup operation archives transaction logs to a SnapInfo directory that resides on the same volume as the transaction logs. To achieve this optimization, SnapManager archives transaction logs to the SnapInfo directory by creating NTFS hard links to the live transaction log files, without physically copying the log file.

After the backup operation completes, Exchange truncates the transaction logs. However, the physical log files on the original location are not deleted, because the NTFS file system detects that other hard links to the files still exist. Instead, the original NTFS hard links to the live transaction log files are removed, and the links to the SnapInfo directory remain intact and available for access during SnapManager restore operations.

SnapManager automatically uses NTFS hard links to archive transaction logs if they reside on the same LUN as the SnapInfo directory.

The entries that are logged in the backup report depend on the archival method that you use. If a transaction log file is archived by performing a file copy operation, for example, the backup report entry is similar to the following:

Log file copying from:
L:\Program Files\Exchsrvr\mdbdata\E000000A.log
Log file copying to:
S:\SME_SnapInfo\EXCH__CLPUBS-WINSRVR3\SG__First Storage Group\CLPUBS-WINSRVR3__recent\Logs\E000000A.log

If the same transaction log file is archived by creating an NTFS hard link instead of performing a file copy operation, the backup report entry is similar to the following:

Log file:
L:\Program Files\Exchsrvr\mdbdata\E000000A.log
Archived to:
S:\SME_SnapInfo\EXCH__CLPUBS-WINSRVR3\SG__First Storage Group\CLPUBS-WINSRVR3__recent\Logs\E000000A.log