DragonFly BSD also include the Dirhash system, developed by Ian Dowse. This system maintains an in-memory hash table to speed up directory lookups. Dirhash alleviates a number of performance problems associated with large directories in UFS. Linux includes a UFS implementation for binary compatibility at the read level with other Unixes, but since there is no standard implementation for the vendor extensions to UFS, Linux does not have full support for writing to UFS. The native Linux ext2 filesystem was inspired by UFS1 but does not support fragments and there are no plans to implement soft updates. (In some 4.4BSD-derived systems, the UFS layer can use an ext2 layer as a container layer, just as it can use FFS and LFS.) NeXTStep, which was BSD-derived, also used a version of UFS. In Apple's Mac OS X, it was available as an alternative to HFS+, their proprietary filesystem. However, as of Mac OS X Leopard, it was no longer possible to install Mac OS X on a UFS-formatted volume. In addition, one cannot upgrade older versions of Mac OS X installed on UFS-formatted volumes to Leopard; upgrading requires reformatting the startup volume. There was a 4 GB file limit for disks formatted as UFS in Mac OS X. As of Mac OS X Lion, UFS support was completely dropped. See also Comparison of file systems References Citations Bibliography External links The Solaris UFS File System, see also USF(sic)/UFS2 format Locality and The Fast File System Berkeley Software Distribution Disk file systems Unix file system technology