The Zettabyte File System (ZFS) is a combined file system and logical volume manager designed to protect against data corruption and support high storage capacities.
The Foundation is sponsoring Matthew Ahrens to develop a “RAID-Z Expansion” feature. This will allow adding an extra disk to an existing RAID-Z group, for example allowing the expansion from a 4-wide RAID-Z1 group into a 5-wide RAID-Z1 group.
This will be accomplished by “reflowing” all existing data, rewriting it into the new arrangement of disks, leaving a new contiguous chunk of free space at the end of the logical RAID-Z group (and thus at the end of each physical disk). The reflowed data will still have the original logical stripe width, i.e. the ratio of data to parity will remain the same, while newly written data will use the new logical stripe width, with an improved data to parity ratio. The reflow happens online, while other zfs and zpool operations are also in progress.
This project is in progress.