Concepts for client restore operations

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The client restore includes the following operations:

No-query restore operations

The client uses two different methods for restore operations: Standard restore (also called classic restore), and no-query restore.

The no-query restore requires less interaction between the client and the server, and the client can use multiple sessions for the restore operation. The no-query restore operation is useful when you restore large file systems on a client with limited memory. The advantage is that no-query restore avoids some processing that can affect the performance of other client applications. In addition, it can achieve a high degree of parallelism by restoring with multiple sessions from the server and storage agent simultaneously.

With no-query restore operations, the client sends a single restore request to the server instead of querying the server for each object to be restored. The server returns the files and directories to the client without further action by the client. The client accepts the data that comes from the server and restores it to the destination named on the restore command.

The no-query restore operation is used by the client only when the restore request meets both of the following criteria:
  • You enter the restore command with a source file specification that has an unrestricted wildcard.
    An example of a source file specification with an unrestricted wildcard is: 
    /home/mydocs/2002/*
    An example of a source file specification with a restricted wildcard is: 
    /home/mydocs/2002/sales.*
  • You do not specify any of the following client options:
    • inactive
    • latest
    • pick
    • fromdate
    • todate
To force classic restore operations, use ?* in the source file specification rather than *. For example: 
/home/mydocs/2002/?*

Controlling resource utilization by a client

You can control the number of mount points (equivalent to drives) allowed to a client by setting the MAXNUMMP parameter on either the UPDATE NODE or REGISTER NODE command.

At the client, the option for resource utilization also affects how many drives (sessions) the client can use. The client option, resource utilization, can be included in a client option set. If the number specified in the MAXNUMMP parameter is too low and there are not enough mount points for each of the sessions, it might not be possible to achieve the benefit of the multiple sessions that are specified in the resource utilization client option.
  • For backup operations, prevent multiple sessions if the client is backing up directly to tape so that data is not spread among multiple volumes. Multiple sessions can be prevented at the client by using a value of 2 for the resource utilization option on the client.
  • For restore operations, set the resource utilization option to one greater than the number of sessions that you want. Use the number of drives that you want that single client to use.
  • With file spaces, a session is limited to processing a single file space. You cannot have multiple backup or restore sessions processing a single file space. However, if you have multiple file spaces on a client, you can have multiple sessions processing those file spaces.

Running multiple commands with backup and restore

You can run multiple commands instead of multiple sessions to speed up the backup and restore of client nodes with critical data.

When you use multi-sessions to back up data, the sessions might be contending for the same underlying hard disk. The contention for resources can cause delays in processing.

An alternative method is to manage backups by starting multiple client commands, where each command backs up a predetermined number of file systems. Using this method, with collocation at the file space level, can improve backup throughput and allow for parallel restore processes across the same hard disk drives.

You must issue multiple commands when you are restoring more than one file space. For example, when you are restoring both a C drive and a D drive on a Windows system you must issue multiple commands.

You can issue the commands one after another in a single session or window, or issue them at the same time from different command windows.

When you enter multiple commands to restore files from a single file space, specify a unique part of the file space in each restore command. Be sure that you do not use any overlapping file specifications in the commands. To display a list of the directories in a file space, issue the QUERY BACKUP command on the client. For example: 
dsmc query backup -dirsonly -subdir=no /usr/

Running multiple sessions on clients for a restore

To use multiple sessions, data for the client must be on multiple sequential access volumes in a file or tape storage pool. Or the data can be contained in a random disk storage pool (with a device class with a device type of DISK). The data for a client usually becomes spread out over some number of volumes over time.

To potentially benefit from multi-session restores, consider collocating client data by group. Collocation by group can cause data for a node to be distributed on more than one volume. The distribution happens while the group's total data is kept on as few volumes as possible.

Restore operations can be restricted on mount points. The MAXNUMMP parameter in the REGISTER NODE or UPDATE NODE command applies to restore operations. The client can restrict the number of sessions, which are based on the combination of the MAXNUMMP value and the client RESOURCEUTILIZATION value. Unlike tape, you can mount FILE volumes in multiple sessions simultaneously, for restore or retrieve operations.

Set the client option for resource utilization to one greater than the number of sessions that you want. Use the number of drives that you want that single client to use. The client option can be included in a client option set.

Issue the restore command so that it results in a no-query restore process.