NDMP DAR Restore of a single file is too slow.

Problem

General Error: NDMP DAR (Direct Access Recovery) Restore of a single file is too slow.

Error Message

Not Applicable: NDMP DAR Restore of a single file is too slow.

Cause

Environment/Conditions:

Here is an example where slow DAR restores have been seen. Using NetBackup to backup 1.38TB or 29 million files on a NetApp Filer with Data ONTAP 7.x. Slow restores should be expected if you attempt to backup too many inodes on a single stream, as it is a DAR limitation.

According to NetApp Solution ID: kb39534, here is how file-level DAR restore works:

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 a while.

Solution

Break the backup into smaller pieces, to restrict the amount of INODE information that DAR has to handle on the restore.

The general expectation is that DAR will greatly increase the restore speed, but this is not always the case. It will depend on the amount and size of the data backed. The larger the INODES during the backup the slower the restore. An NDMP restore must read the inode map and that is more than likely what is causing the issue.

With DAR, NDMP needs to do quite a bit of pre-processing and then to read the bitmap. It could be that the pre-processing and the bitmap read are taking all this time. Break this huge backup down into smaller qtrees and DAR restore would work much quicker.

Please consult Filer Vendor for more information.