of magnetic head could be only in one position at a time to read data from platter while fragmentation does no effect on SSD drives. Releases of ONTAP since 7.3.1 have included a number of techniques to optimize spatial data layout such as the reallocate command to perform scheduled and manual defragmentation, and the Write after Reading volume option which detects and automatically corrects suboptimal data access patterns caused by spatial fragmentation. Releases of ONTAP 8.1.1 include other techniques to automatically optimize contiguous free-space within the filesystem which also helps to maintain optimal data layouts for most data access patterns. Before 7G, the wafl scan reallocate command would need to be invoked from an advanced privilege level and could not be scheduled. Releases of ONTAP since 9.1 have included a number of techniques to optimize SSD usage such as Inline Data Compaction (in 9.1), starting with ONTAP 9.2 FabricPool functionality for automatic tiering of cold data to slow S3 storage and back if needed for SSD aggregates, and Cross Volume Deduplication within an aggregate with maximum of 800TiB for each aggregate. Snapshots WAFL supports snapshots, which are read-only copies of a file system. Snapshots are created by performing the same operations that are performed in a consistency point, but, instead of updating the root inode corresponding to the current state of the file system, saving a copy of the root inode. As all data and metadata in a file system can be found from the root inode, all data and metadata in a file system, as of the time when the snapshot is created, can be found from the snapshot's copy of the root inode. No other data needs to be copied to create a snapshot. Blocks are allocated when written using a block map, which keeps track of which blocks are in use and which blocks are free. An entry in the block map contains a bit indicating whether the block is in use in the current version of the file system and several bits, one per snapshot, indicating whether the block is in use in the snapshot. This ensures that data in a snapshot is not overwritten until the snapshot is deleted. Using block map all new writes and rewrites are written to new empty blocks, WAFL only reports that block rewrite successful, but no rewrites actually occur, this approach called Redirect-on-write (ROW) technique. ROW is much faster