of a chunk has fallen beneath the set number). All this metadata is kept current by the Master server periodically receiving updates from each chunk server ("Heart-beat messages"). Permissions for modifications are handled by a system of time-limited, expiring "leases", where the Master server grants permission to a process for a finite period of time during which no other process will be granted permission by the Master server to modify the chunk. The modifying chunkserver, which is always the primary chunk holder, then propagates the changes to the chunkservers with the backup copies. The changes are not saved until all chunkservers acknowledge, thus guaranteeing the completion and atomicity of the operation. Programs access the chunks by first querying the Master server for the locations of the desired chunks; if the chunks are not being operated on (i.e. no outstanding leases exist), the Master replies with the locations, and the program then contacts and receives the data from the chunkserver directly (similar to Kazaa and its supernodes). Unlike most other file systems, GFS is not implemented in the kernel of an operating system, but is instead provided as a userspace library. Interface The Google File System does not provide a POSIX interface. Files are organized hierarchically in directories and identified by pathnames. The file operations such as create, delete, open, close, read, write are supported. It supports Record Append which allows multiple clients to append data to the same file concurrently and atomicity is guaranteed. Performance Deciding from benchmarking results, when used with relatively small number of servers (15), the file system achieves reading performance comparable to that of a single disk (80–100 MB/s), but has a reduced write performance (30 MB/s), and is relatively slow (5 MB/s) in appending data to existing files. The authors present no results on random seek time. As the master node is not directly involved in data reading (the data are passed from the chunk server directly to the reading client), the read rate increases significantly with the number of chunk servers, achieving 583 MB/s for 342 nodes. Aggregating multiple servers also allows big capacity, while it is somewhat reduced by storing data in three independent locations (to provide redundancy). See also Bigtable Cloud storage CloudStore Fossil, the native file system of Plan 9 GPFS IBM's General Parallel File System GFS2 Red Hat's Global Filesystem 2 Hadoop and its "Hadoop Distributed File System" (HDFS), an