The active log mirror is a copy of the active log that can be used if the active log files cannot be read. All changes made to the active log are also written to the log mirror. There can be only one active log mirror.
Mirroring the active log can protect the database when a hardware failure occurs on the device where the active log is stored. Mirroring the active log provides another level of protection in addition to placing the active log on hardware that has high-availability features. Creating a log mirror is optional but recommended. Place the active log directory and the log mirror directory on different physical devices. If you increase the size of the active log, the log mirror size is increased automatically.
Mirroring the log can affect performance, because of the doubled I/O activity that is required to maintain the mirror. The additional space that the log mirror requires is another factor to consider.
You can create the log mirror during initial configuration of a new or upgraded server. If you use the DSMSERV LOADFORMAT utility instead of the wizard to configure the server, specify the MIRRORLOGDIRECTORY parameter. If the log mirror directory is not created at that time, you can create it later by specifying the MIRRORLOGDIRECTORY option in the server options file, dsmserv.opt.