up to 4K. Access to a data record requires two levels of indirection, where the file's directory entry (called a File Status Table (FST) entry) points to blocks containing a list of addresses of the individual records. AS/400 file system Data on the AS/400 and its successors consists of system objects mapped into the system virtual address space in a single-level store. Many types of objects are defined including the directories and files found in other file systems. File objects, along with other types of objects, form the basis of the AS/400's support for an integrated relational database. Other file systems The Prospero File System is a file system based on the Virtual System Model. The system was created by Dr. B. Clifford Neuman of the Information Sciences Institute at the University of Southern California. RSRE FLEX file system - written in ALGOL 68 The file system of the Michigan Terminal System (MTS) is interesting because: (i) it provides "line files" where record lengths and line numbers are associated as metadata with each record in the file, lines can be added, replaced, updated with the same or different length records, and deleted anywhere in the file without the need to read and rewrite the entire file; (ii) using program keys files may be shared or permitted to commands and programs in addition to users and groups; and (iii) there is a comprehensive file locking mechanism that protects both the file's data and its metadata. Limitations Converting the type of a file system It may be advantageous or necessary to have files in a different file system than they currently exist. Reasons include the need for an increase in the space requirements beyond the limits of the current file system. The depth of path may need to be increased beyond the restrictions of the file system. There may be performance or reliability considerations. Providing access to another operating system which does not support the existing file system is another reason. In-place conversion In some cases conversion can be done in-place, although migrating the file system is more conservative, as it involves a creating a copy of the data and is recommended. On Windows, FAT and FAT32 file systems can be converted to NTFS via the convert.exe utility, but not the reverse. On Linux, ext2 can be converted to ext3 (and converted back), and ext3 can be converted to ext4 (but not