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Chapter 5: z/OS Distributed File Service... > zFS introduction - Pg. 230

5.1 zFS introduction The z/OS Distributed File Service (DFS) zSeries File System (zFS) is a z/OS UNIX file system that can be used in addition to the Hierarchical File System (HFS). zFS provides significant performance gains in accessing files approaching 8K in size that are frequently accessed and updated. The access performance of smaller files is equivalent to that of HFS. zFS provides reduced exposure to loss of updates by writing data blocks asynchronously and not waiting for a sync interval. zFS is a journaling file system. It logs metadata updates, then if a system failure occurs, zFS replays the log when it comes back up to ensure that the file system is consistent. zFS is a Physical File System (PFS) that is started by UNIX System Services (USS) during IPL. A physical file system is the part of the operating system that handles the actual storage and manipulation of data on a storage medium. 5.2 Application programming interfaces (APIs) zFS file systems contain files and directories that can be accessed with the z/OS hierarchical file system application programming interfaces on the z/OS operating system as follows: An application interface composed of C interfaces, some of which are managed within the C Run-Time Library (RTL), while others access kernel interfaces to perform authorized system functions on behalf of the unauthorized caller An interactive z/OS shell interface used by shell users