and the file system transfers data from the media to the buffer. A runtime library routine may sometimes allow the user program to define a record based on a library call specifying a length. When the user program reads the data, the library retrieves data via the file system and returns a record. Some file systems allow the specification of a fixed record length which is used for all writes and reads. This facilitates locating the nth record as well as updating records. An identification for each record, also known as a key, makes for a more sophisticated file system. The user program can read, write and update records without regard to their location. This requires complicated management of blocks of media usually separating key blocks and data blocks. Very efficient algorithms can be developed with pyramid structures for locating records. Using a file system Utilities, language specific run-time libraries and user programs use file system APIs to make requests of the file system. These include data transfer, positioning, updating metadata, managing directories, managing access specifications, and removal. Multiple file systems within a single system Frequently, retail systems are configured with a single file system occupying the entire storage device. Another approach is to partition the disk so that several file systems with different attributes can be used. One file system, for use as browser cache or email storage, might be configured with a small allocation size. This keeps the activity of creating and deleting files typical of browser activity in a narrow area of the disk where it will not interfere with other file allocations. Another partition might be created for the storage of audio or video files with a relatively large block size. Yet another may normally be set read-only and only periodically be set writable. A third approach, which is mostly used in cloud systems, is to use "disk images" to house additional file systems, with the same attributes or not, within another (host) file system as a file. A common example is virtualization: one user can run an experimental Linux distribution (using the ext4 file system) in a virtual machine under his/her production Windows environment (using NTFS). The ext4 file system resides in a disk image, which is treated as a file (or multiple files, depending on the hypervisor and settings) in the NTFS host file system. Having multiple file systems on a single system has