NDMP DAR (Direct Access Recovery) Restores are very slow (4039277)

NDMP Restores are very slow when backups were taken with DAR (Direct Access Recovery)

This article comes from the Netapp article on NDMP File-Level DAR and how it works, and why they can be slow.

This is the process that happens when file level DAR is used:

File level DAR backup

1. When DAR is enabled for backup for both the filer Network Data Management Protocol (NDMP) and the backup software, the software requests the file(s) for the backup job.
2. The filer then starts the five phases of the backup process. See '3011854: NDMP / Dump phases description' for more information.
3. Once these phases are done and the job is complete, the information is stored on the internal database of the backup software for later recovery.

File level DAR restore

1. When the file is selected by the backup software for restore, the request is sent to the filer.
2. The filer receives the filename, and the offset map created from the restore and reads the appropriate part of the backup tape and compares the offset found there.
3. If the offset matches, the filer then seeks through the tape to locate the directory that the file is contained in. This could span the entire tape or even multiple tapes until the offset is reached.
4. Once the directory is reached, the entire directory structure is read and compared.
5. After confirmation, the selected file is located and restored to the filer's selected location along with the metadata.

Depending on the number of files in the directory, position of directory and file on the tape, and the number of tapes needing to be read, this process can take awhile.