Google File System

Google File System
Design

GFS is optimized for Google's core data storage and usage needs (primarily the search engine
), which can generate enormous amounts of data that needs to be retained;<sup>[2]

Google File System grew out of an earlier Google effort, "BigFiles", developed by Larry Page
and Sergey Brin
in the early days of Google, while it was still located in Stanford
.[2]

Files are divided into chunks of 64 megabytes
, which are only extremely rarely overwritten, or shrunk; files are usually appended to or read. It is also designed and optimized to run on Google's computing clusters, the nodes of which consist of cheap, "commodity" computers, which means precautions must be taken against the high failure rate of individual nodes and the subsequent data loss. Other design decisions select for high data throughputs
, even when it comes at the cost of latency
.</sup>

The nodes are divided into two types: one Master
node and a large number of Chunkservers
. Chunkservers store the data files, with each individual file broken up into fixed size chunks (hence the name) of about 64 megabytes
,<sup>[3]

similar to clusters or sectors in regular file systems. Each chunk is assigned a unique 64-bit label, and logical mappings of files to constituent chunks are maintained. Each chunk is replicated several times throughout the network, with the minimum being three, but even more for files that have high demand or need more redundancy.</sup>

The Master server doesn't usually store the actual chunks, but rather all the metadata
associated with the chunks, such as the tables mapping the 64-bit labels to chunk locations and the files they make up, the locations of the copies of the chunks, what processes are reading or writing to a particular chunk, or taking a "snapshot" of the chunk pursuant to replicating it (usually at the instigation of the Master server, when, due to node failures, the number of copies 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 modified 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 (if there are no outstanding leases), 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
).

As opposed to many filesystems, GFS is not implemented in the kernel of an operating system
, but is instead provided as a userspace library.