Preliminary Information for Creating Trans Metadevices

• A trans metadevice enables UFS logging, which is the process of recording UFS updates in a log before the updates are applied to the UNIX file system. A trans metadevice can increase overall file system availability after reboot, because it reduces the amount of time fsck(1M) has to run when the system reboots.

• The system must contain at least three state database replicas before you can create a trans metadevice.

• The trans metadevice normally has two devices: the master device and the logging device. The master contains the file system that is being logged. The logging device contains the log and can be shared by several file systems. It is a sequence of records, each of which describes a change to a file system. Both the master device and the logging device can be a slice or a metadevice.

• Though logs can be shared between file systems, heavily-used file systems should have their own logging device.

• Small file systems with mostly read operations probably do not need to be logged.

• Any UFS, except root (/), can be logged.

• Even if you don't have an available slice for a logging device, you can still set up a trans metadevice without a logging device. This is useful if you plan to enable logging on exported file systems, but do not have an available slice for the logging device at this time.

• Before creating trans metadevices, identify the slices or metadevice to be used as the master devices and logging devices.

• Avoid placing logs on heavily-used disks.

• Do not use a RAID5 metadevice as a logging device. Instead, use a mirror for data redundancy.

• Logs (the logging device) can be placed on a slice that already contains a state database replica.

• Plan on using one megabyte of log space as a minimum, and an additional one megabyte of log space per 100 megabytes of file system data, up to a maximum log size of 64 Mbytes. Logs greater than 64 Mbytes waste space.

• The master devices and logging devices of the same trans metadevice should be located on separate drives and possibly separate controllers.

Mirroring logging devices is strongly recommended. Losing the data in a logging device because of device errors can leave a file system in an inconsistent state which fsck(1M) may not be able to fix without user intervention. Using a mirror for the master device is a good idea to ensure data redundancy.