See About SmartTier.
Volume sets have the following limitations:
The first volume (index 0) in a volume set must be larger than the sum of the total volume size divided by 4000, the size of the VxFS intent log, and 1MB. Volumes 258 MB or larger should always suffice.
Volume sets can be used in place of volumes with the following vxsnap operations on instant snapshots: addmir, dis, make, prepare, reattach, refresh, restore, rmmir, split, syncpause, syncresume, syncstart, syncstop, syncwait, and unprepare. The third-mirror break-off usage model for full-sized instant snapshots is supported for volume sets provided that sufficient plexes exist for each volume in the volume set.
A full-sized snapshot of a volume set must itself be a volume set with the same number of volumes and the same volume index numbers as the parent. The corresponding volumes in the parent and snapshot volume sets are also subject to the same restrictions as apply between standalone volumes and their snapshots.