After a trans metadevice is configured, it can be used just as if it were a physical slice. A trans metadevice can be used as a block device (up to 2 Gbytes) or a raw device (up to 1 Tbyte). A UFS file system can be created on the trans metadevice if the master device doesn't already have a file system.
A logging device or a master device can be a physical slice or a metadevice. For reliability and availability, however, use mirrors for logging devices. A device error on a physical logging device could cause data loss. You can also use mirrors or RAID5 metadevices as master devices.
A minimum of 1 Mbyte. (Larger logs permit more simultaneous file-system transactions.) The maximum log size is 1 Gbyte. 1 Mbyte worth of log per 1 Gbyte of file system is a recommended minimum. 1 Mbyte worth of log per 100 Mbyte of file system is a recommended "average." Unfortunately, there are no hard and fast rules. The best log size varies with an individual system's load and configuration. However, a log larger than 64 Mbytes will rarely be used. Fortunately, log sizes can be changed without too much work.
Generally, log your largest UFS file systems and the UFS file system whose data changes most often. It is probably not necessary to log small file systems with mostly read activity.
Which file systems should always have separate logs?
All logged file systems can shared the same log. For better performance, however, file systems with the heaviest loads should have separate logs.
You must disable logging for /usr, /var, /opt, or any other file systems used by the system during a Solaris upgrade or installation when installing or upgrading software on a Solaris system.
Place logs on mirrors, unused slices, or slices that contain the state database replicas. A device error on a physical logging device (a slice) can cause data loss.
What if no slice is available for the logging device?
You can still configure a trans metadevice. This may be useful if you plan to log exported file systems when you do not have a spare slice for the logging device. When a slice is available, you only need to attach it as a logging device. For instructions, see Solstice DiskSuite 4.2.1 User's Guide.
Yes, a logging device can be shared between file systems, though heavily-used file systems should have their own logging device. 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.