Logs (logging devices) are typically accessed frequently. For best performance, avoid placing them on heavily-used disks. You may also want to place logs in the middle of a disk, to minimize the average seek times when accessing the log.
The log device and the master device of the same trans metadevice should be located on separate drives and possibly separate controllers to help balance the I/O load.
Sharing logs: trans metadevices can share log devices. However, if a file system is heavily used, it should have a separate log. The disadvantage to sharing a logging device is that certain errors require that all file systems sharing the logging device must be checked with the fsck(1M) command.
The larger the log size, the better the performance. Larger logs allow for greater concurrency (more simultaneous file system operations per second).
The absolute minimum size for a logging device is 1 Mbyte. A good average for performance is 1 Mbyte of log space for every 100 Mbyte of file system space. A recommended minimum is 1 Mbyte of log for every 1 Gbyte of file system space.
Assume you have a 4 Gbyte file system. What are the recommended log sizes?
For good performance, a size of 40 Mbyte is recommended (1 Mbyte log / 100 Mbyte file system).
A recommended minimum is 4 Mbyte (1 Mbyte log/1 Gbyte file system).
The absolute minimum is 1 Mbyte.
It is strongly recommended that you mirror all logs. It is possible to lose the data in a log because of device errors. If the data in a log is lost, it can leave a file system in an inconsistent state which fsck may not be able to repair without user intervention.