About the mirror snapshot type
Unlike a copy-on-write, a mirror is a complete data copy stored on a separate disk, physically independent of the original. Every change or write to the data on the primary disk is also made to the copy on the secondary disk. The copy is a "mirror" image of the source data.
As in a copy-on-write, transactions are allowed to finish and new I/O on the primary disk is briefly halted. When the mirror image is brought up-to-date with the source, the mirror is split from the primary. After the mirror is split, new changes can be made to the primary but not to the mirror. The mirror can now be backed up (see next diagram).
If the mirror is to be used again it must be brought up-to-date with the primary volume (synchronized). During synchronization, the changes that were made to the primary volume - while the mirror was split - are written to the mirror.
Since mirroring requires a complete copy of the primary on a separate device (same size as the primary), it consumes more disk space than copy-on-write.