back), and both ext3 and ext4 can be converted to btrfs, and converted back until the undo information is deleted. These conversions are possible due to using the same format for the file data itself, and relocating the metadata into empty space, in some cases using sparse file support. Migrating to a different file system Migration has the disadvantage of requiring additional space although it may be faster. The best case is if there is unused space on media which will contain the final file system. For example, to migrate a FAT32 file system to an ext2 file system. First create a new ext2 file system, then copy the data to the file system, then delete the FAT32 file system. An alternative, when there is not sufficient space to retain the original file system until the new one is created, is to use a work area (such as a removable media). This takes longer but a backup of the data is a nice side effect. Long file paths and long file names In hierarchical file systems, files are accessed by means of a path that is a branching list of directories containing the file. Different file systems have different limits on the depth of the path. File systems also have a limit on the length of an individual filename. Copying files with long names or located in paths of significant depth from one file system to another may cause undesirable results. This depends on how the utility doing the copying handles the discrepancy. See also Comparison of file systems Disk quota List of file systems List of Unix commands Directory structure Shared resource Distributed file system Distributed Data Management Architecture File manager File system fragmentation Filename extension Global file system Object storage Computer data storage Storage efficiency Virtual file system Notes References Sources Further reading Books Prabhakaran, Vijayan (2006). IRON File Systems. PhD dissertation, University of Wisconsin-Madison. Online Benchmarking Filesystems (outdated) by Justin Piszcz, Linux Gazette 102, May 2004 Benchmarking Filesystems Part II using kernel 2.6, by Justin Piszcz, Linux Gazette 122, January 2006 Filesystems (ext3, ReiserFS, XFS, JFS) comparison on Debian Etch 2006 Interview With the People Behind JFS, ReiserFS & XFS Journal File System Performance (outdated): ReiserFS, JFS, and Ext3FS show their merits on a fast RAID appliance Journaled Filesystem Benchmarks (outdated): A comparison of ReiserFS, XFS, JFS, ext3 & ext2 Large List of File System Summaries (most