Storage appliance vendor Ctera has announced that the Next3 filesystem is now available. Next3 is based on Ext3 and adds lightweight snapshots to the former Linux standard filesystem (which has now largely been superseded by Ext4). The data format stored on disk is identical: A Next3 filesystem can also be mounted as Ext3 – although it will be read-only and can’t be written to; existing Ext3 filesystems can be converted to Next3.
Snapshots are stored in special, filesystem-sized sparse files and are created by moving deleted or overwritten data blocks into the sparse file. When accessing the sparse file, all files are available in the version they were in when the snapshot was created – any holes in the snapshot file will automatically be filled in with the unchanged data in the original filesystem. Find more details about Next3 in an article at LWN and a paper released by the developers. Next3 is available under the GPL 2.
The Next3 developers’ target is the integration of their filesystem into the Linux kernel. However, this could prove difficult: Ext4 has already been established as the next development step to follow Ext3, but Next3 develops Ext3 in a different direction which is not compatible with Ext4. Porting the changes in Next3 to Ext4 is quite involved, because Ext4 manages the data on disk via extents rather than block lists.