Logging Device Trade-offs

• 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.