Without DAR (Direct Access Recovery), a restore operation of a NetApp must read the entire data set from the backup media. The NDMP agent on the NetApp server then only restores requested the portions of the backup while it is processing the entire data stream. The purpose of the DAR feature is to change this functionality such that the restore operation quickly seeks directly to the position on the backup media where the requested data resides, and then only reads that data to complete the restore operation.
The DAR restore should be much faster if only a small portion of the backup set is being restored because only that data has to be read. In practice the read portion of the process when using DAR is extremely slow when coming from tape. The NetApp NDMP server is reading relatively small chunks of data from the tape (1MB). In between each request for data, the tape underruns and has to reposition. This results in a shoe-shining action on the tape which greatly impacts the overall performance of the reads.
When DAR is turned off for the restore operation, the restore has to read all of the media to complete the restore operation. One of the side effects of this is that, if the data to be restored is near the front of the data set then the file being restored may be available long before the restore operation itself finishes reading the rest of the media. However, non-DAR restore operations will outperform DAR restore operations when larger data sets are being restored.