Venti is a network storage system that permanently stores data blocks. A 160-bit SHA-1 hash of the data (called ''score'' by Venti) acts as the address of the data. This enforces a ''write-once'' policy since no other data block can be found with the same address: the addresses of multiple writes of the same data are identical, so it is highly likely that duplicate data is easily identified and the data block is stored only once. Data blocks cannot be removed, making it ideal for permanent or backup storage. Venti is typically used with

Fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserve ...

to provide a file system with permanent snapshots.

History

Venti was designed and implemented by Sean Quinlan and Sean Dorward at

Bell Labs Nokia Bell Labs, commonly referred to as ''Bell Labs'', is an American industrial research and development company owned by Finnish technology company Nokia. With headquarters located in Murray Hill, New Jersey, Murray Hill, New Jersey, the compa ...

. It appeared in the Plan 9 distribution in 2002. Development has been continued by Russ Cox who has reimplemented most of the server, written a library for creating datastructures (files, directories and meta-data) to store in Venti and implemented optimizations. Venti is available both in the Plan 9 distribution and for many Unix-like operating systems as part of Plan 9 from User Space. Venti is included as part of Inferno with accompanying modules for access. There is a Go set of programs to build your own Venti servers. Included are examples using different kinds of backend storage.

Details

Venti is a user space daemon.Lukkien, Mechiel. Venti Analysis and Memventi Implementation. Thesis. University of Twente, 2007. N.p.: n.p., n.d. University of Twente Theses Repository. Web. 13 Oct. 2014. . Clients connect to Venti over TCP and communicate using a simple RPC-protocol. The most important messages of the protocol are listed below. Note that there is no message to delete an address or modify data at a given address. * ''read(score, type)'', returns the data identified by ''score'' and ''type'' * ''write(data, type)'', stores ''data'' at the address calculated by SHA-1 hashing ''data'', combined with ''type''. The data block stored by Venti must be greater than 512 bytes in length and smaller than 56 kilobytes. So, if a Venti user/client wants to store larger data blocks, it has to make a datastructure (which can be stored in Venti). For example,

uses hash trees to store large files. Venti itself is not concerned with the contents of a data block; it does however store the ''type'' of a data block. The design of Venti has the following consequences: * Since writes are permanent, the file system is append-only (which allows for a simple implementation with lower chance of data-destroying bugs); no file system fragmentation occurs. * Clients can verify the correctness of the server: the score of the returned data should be the same as the address requested. Since SHA-1 is a cryptographically secure hash, it is computationally infeasible to fabricate data. * Data cannot be overwritten. If an ''address'' is already present, the ''data'' is already present. * There is little need for user authentication: Data cannot be deleted, and can be read only if the score is known. The only potential problem is a user filling up the disks. * Data can be compressed without making the disk structure complicated. The data blocks are stored on

hard drive A hard disk drive (HDD), hard disk, hard drive, or fixed disk is an electro-mechanical data storage device that stores and retrieves digital data using magnetic storage with one or more rigid rapidly rotating hard disk drive platter, pla ...

s. The disks making up the available storage, typically a

RAID RAID (; redundant array of inexpensive disks or redundant array of independent disks) is a data storage virtualization technology that combines multiple physical Computer data storage, data storage components into one or more logical units for th ...

, is called the ''data log''. This data log is split up in smaller pieces called ''arenas'', which are sized so they can be written to other media such as CD/ DVD or

magnetic tape Magnetic tape is a medium for magnetic storage made of a thin, magnetizable coating on a long, narrow strip of plastic film. It was developed in Germany in 1928, based on the earlier magnetic wire recording from Denmark. Devices that use magnetic ...

. Another set of hard drives is used for the index, which maps scores to addresses in the data log. The data structure used for the index is a

hash table In computer science, a hash table is a data structure that implements an associative array, also called a dictionary or simply map; an associative array is an abstract data type that maps Unique key, keys to Value (computer science), values. ...

with fixed-sized buckets. Venti relies on the scores to be randomly distributed so buckets do not fill up. Since each lookup costs one disk

seek time Higher performance in hard disk drives comes from devices which have better performance characteristics. These performance characteristics can be grouped into two categories: #Access time, access time and #Data transfer rate, data transfer time (o ...

, an index usually consists of multiple hard drives with low access time.

Usage

The Venti server may be used by clients in several ways. The Plan 9 operating system makes use of Venti for daily archival snapshots of the file system. These copies of the main filesystem can be mounted as a filetree of full copies organized by date. The utility programs 'vac' and 'unvac' can be used to store and retrieve data from a Venti server in the form of individual files or as a directory and its contents. 'Vacfs' allows browsing of the data associated with a vac score without full retrieval of all remotely stored data. Data and index scores can be duplicated between Venti servers using 'rdarena' and 'wrarena'.

Plan 9 from Bell Labs Plan 9 from Bell Labs is a distributed operating system which originated from the Computing Science Research Center (CSRC) at Bell Labs in the mid-1980s and built on UNIX concepts first developed there in the late 1960s. Since 2000, Plan 9 has ...

, Plan 9 from User Space, Inferno and any other clients that implement the Venti protocol can all be used interchangeably to store and retrieve data.

Hash collisions

A basic principle of

information theory Information theory is the mathematical study of the quantification (science), quantification, Data storage, storage, and telecommunications, communication of information. The field was established and formalized by Claude Shannon in the 1940s, ...

is the

pigeonhole principle In mathematics, the pigeonhole principle states that if items are put into containers, with , then at least one container must contain more than one item. For example, of three gloves, at least two must be right-handed or at least two must be l ...

, which states that if set A contains more values than set B, then for any function that maps A to B there will be members of B that are associated with more than one member of set A. In the case of Venti, the set of possible SHA-1 hashes is obviously smaller than the set of all possible blocks that could be stored in the filesystem, and thus a hash collision is possible. The risk of accidental hash collision in a 160-bit hash is very small, even for exabytes of data. Historically, however, many hash functions become increasingly vulnerable to malicious hash collisions due to both cryptographic and computational advances."HASH COLLISION Q&A." Cryptography Research. Rambus, n.d. Web. 12 Jan. 2010. . Venti does not address the issue of hash collisions; as of this time, it is still computationally infeasible to find collisions in SHA-1, but it may become necessary for Venti to switch to a different hash function at some point in the future. On 23 February 2017, Google announced the SHAttered attack, in which they generated two different PDF files with the same SHA-1 hash in roughly 2^63.1 SHA-1 evaluations.

References

{{reflist

External links