Question & Answer
Restore processing is done in one of two models, "Classic" or "No Query Restore (NQR)". This can be important when running Tivoli Storage Manager restores.
When using the Tivoli Storage Manager client to restore data from the server, either the "classic restore" or "no query restore" (NQR) technique can be used to request the files from the Tivoli Storage Manager server.
The Tivoli Storage Manager restore processing uses the No-Query-Restore technique when an unrestricted wildcard source file specification is entered on the restore command. The classic restore technique is used if any of the following options are specified:
When the classic restore technique is invoked, the Tivoli Storage Manager client does the following:
- queries the Tivoli Storage Manager server for names of eligible files,
- sorts the list of file names into "restore order" sequence (i.e. by tape volids & tape locations in order to avoid unnecessary tape mounts & tape positioning during the restore operation), and
- requests the files from the server in groups (# of files in the group are governed by TXNBytelimit setting on the client & TXNGroupmax setting on the server).
The Classic restore technique has worked well for all restore options for many years. However, as the size of client file systems grew larger and larger, maintaining a list of files to be restored in client memory became cumbersome. We frequently encounter Windows and Unix file servers with 2 to 3 million files in each file system. If these machines contain only a limited amount of RAM, the Tivoli Storage Manager client restore function of an entire file system could cause an excessive amount of system paging and affect the performance of other applications on the same client machine.
The NQR reduces the memory requirement on Tivoli Storage Manager clients and is optimized for restoring large file systems. The NQR code evaluates every file on every volume on which the client has data, and starts sending qualified files to the client as soon as it has eligible candidates while looking for more candidate files.
However, some Tivoli Storage Manager implementations with large client file systems have reported performance problems when the NQR technique is used and only a few files qualify as restore candidates. In one case, it took the Tivoli Storage Manager server one hour to search through its database tables before eligible files were sent to the client. Using the "-latest" option as part of the restore command (i.e. forcing the use of classic restore technique) it was possible to restore the same few files in two minutes.
Currently there is no fix for this issue. Along with the circumventions mentioned above you can force the use of a classic restore by:
- Specify the DISABLENQR option in the client dsm.opt option file, for more information:
- Use the following wildcard syntax in the restore file specification:
- Select the "Disable No Query Restore method" check box in the client GUI.
17 June 2018