venti – archival storage server

Venti is a block storage server intended for archival data. In a Venti server, the SHA1 hash of a block's contents acts as the block identifier for read and write operations. This approach enforces a write–once policy, preventing accidental or malicious destruction of data. In addition, duplicate copies of a block are coalesced, reducing the consumption of storage and simplifying the implementation of clients.

This manual page documents the basic concepts of block storage using Venti as well as the Venti network protocol.

Venti(1) documents some simple clients. Vac(1) and vacfs(4) are more complex clients.

Venti(2) describes a C library interface for accessing Venti servers and manipulating Venti data structures.

Venti(8) describes the programs used to run a Venti server.

The SHA1 hash that identifies a block is called its score. The score of the zero–length block is called the zero score.

Scores may have an optional label: prefix, typically used to describe the format of the data. For example, vac(1) uses a vac: prefix.

Files and Directories
Venti accepts blocks up to 56 kilobytes in size. By convention, Venti clients use hash trees of blocks to represent arbitrary–size data files. The data to be stored is split into fixed–size blocks and written to the server, producing a list of scores. The resulting list of scores is split into fixed–size pointer blocks (using only an integral number of scores per block) and written to the server, producing a smaller list of scores. The process continues, eventually ending with the score for the hash tree's top–most block. Each file stored this way is summarized by a VtEntry structure recording the top–most score, the depth of the tree, the data block size, and the pointer block size. One or more VtEntry structures can be concatenated and stored as a special file called a directory. In this manner, arbitrary trees of files can be constructed and stored.

Scores passed between programs conventionally refer to VtRoot blocks, which contain descriptive information as well as the score of a directory block containing a small number of directory entries.

Conventionally, programs do not mix data and directory entries in the same file. Instead, they keep two separate files, one with directory entries and one with metadata referencing those entries by position. Keeping this parallel representation is a minor annoyance but makes it possible for general programs like venti/copy (see venti(1)) to traverse the block tree without knowing the specific details of any particular program's data.

Block Types
To allow programs to traverse these structures without needing to understand their higher–level meanings, Venti tags each block with a type. The types are:

VtDataType       000    data
VtDataType+1     001    scores of VtDataType blocks
VtDataType+2     002    scores of VtDataType+1 blocks
VtDirType        010    VtEntry structures
VtDirType+1      011    scores of VtDirType blocks
VtDirType+2      012    scores of VtDirType+1 blocks
VtRootType       020    VtRoot structure

The octal numbers listed are the type numbers used by the commands below. (For historical reasons, the type numbers used on disk and on the wire are different from the above. They do not distinguish VtDataType+n blocks from VtDirType+n blocks.)

Zero Truncation
To avoid storing the same short data blocks padded with differing numbers of zeros, Venti clients working with fixed–size blocks conventionally `zero truncate' the blocks before writing them to the server. For example, if a 1024–byte data block contains the 11–byte string `hello world' followed by 1013 zero bytes, a client would store only the 11–byte block. When the client later read the block from the server, it would append zero bytes to the end as necessary to reach the expected size.

When truncating pointer blocks (VtDataType+n and VtDirType+n blocks), trailing zero scores are removed instead of trailing zero bytes.

Because of the truncation convention, any file consisting entirely of zero bytes, no matter what its length, will be represented by the zero score: the data blocks contain all zeros and are thus truncated to the empty block, and the pointer blocks contain all zero scores and are thus also truncated to the empty block, and so on up the hash tree.

Network Protocol
A Venti session begins when a client connects to the network address served by a Venti server; the conventional address is tcp!server!venti (the venti port is 17034). Both client and server begin by sending a version string of the form venti–versions–comment\n. The versions field is a list of acceptable versions separated by colons. The protocol described here is version 02. The client is responsible for choosing a common version and sending it in the VtThello message, described below.

After the initial version exchange, the client transmits requests (T–messages) to the server, which subsequently returns replies (R–messages) to the client. The combined act of transmitting (receiving) a request of a particular type, and receiving (transmitting) its reply is called a transaction of that type.

Each message consists of a sequence of bytes. Two–byte fields hold unsigned integers represented in big–endian order (most significant byte first). Data items of variable lengths are represented by a one–byte field specifying a count, n, followed by n bytes of data. Text strings are represented similarly, using a two– byte count with the text itself stored as a UTF–encoded sequence of Unicode characters (see utf(6)). Text strings are not NUL–terminated: n counts the bytes of UTF data, which include no final zero byte. The NUL character is illegal in text strings in the Venti protocol. The maximum string length in Venti is 1024 bytes.

Each Venti message begins with a two–byte size field specifying the length in bytes of the message, not including the length field itself. The next byte is the message type, one of the constants in the enumeration in the include file <venti.h>. The next byte is an identifying tag, used to match responses to requests. The remaining bytes are parameters of different sizes. In the message descriptions, the number of bytes in a field is given in brackets after the field name. The notation parameter[n] where n is not a constant represents a variable–length parameter: n[1] followed by n bytes of data forming the parameter. The notation string[s] (using a literal s character) is shorthand for s[2] followed by s bytes of UTF–8 text. The notation parameter[] where parameter is the last field in the message represents a variable–length field that comprises all remaining bytes in the message.

All Venti RPC messages are prefixed with a field size[2] giving the length of the message that follows (not including the size field itself). The message bodies are:
VtThello tag[1] version[s] uid[s] strength[1] crypto[n] codec[n]
VtRhello tag[1] sid[s] rcrypto[1] rcodec[1]
VtTping tag[1]
VtRping tag[1]
VtTread tag[1] score[20] type[1] pad[1] count[2]
VtRread tag[1] data[]
VtTwrite tag[1] type[1] pad[3] data[]
VtRwrite tag[1] score[20]
VtTsync tag[1]
VtRsync tag[1]
VtRerror tag[1] error[s]
VtTgoodbye tag[1]

Each T–message has a one–byte tag field, chosen and used by the client to identify the message. The server will echo the request's tag field in the reply. Clients should arrange that no two outstanding messages have the same tag field so that responses can be distinguished.

The type of an R–message will either be one greater than the type of the corresponding T–message or Rerror, indicating that the request failed. In the latter case, the error field contains a string describing the reason for failure.

Venti connections must begin with a hello transaction. The VtThello message contains the protocol version that the client has chosen to use. The fields strength, crypto, and codec could be used to add authentication, encryption, and compression to the Venti session but are currently ignored. The rcrypto, and rcodec fields in the VtRhello response are similarly ignored. The uid and sid fields are intended to be the identity of the client and server but, given the lack of authentication, should be treated only as advisory. The initial hello should be the only hello transaction during the session.

The ping message has no effect and is used mainly for debugging. Servers should respond immediately to pings.

The read message requests a block with the given score and type. Use vttodisktype and vtfromdisktype (see venti(2)) to convert a block type enumeration value (VtDataType, etc.) to the type used on disk and in the protocol. The count field specifies the maximum expected size of the block. The data in the reply is the block's contents.

The write message writes a new block of the given type with contents data to the server. The response includes the score to use to read the block, which should be the SHA1 hash of data.

The Venti server may buffer written blocks in memory, waiting until after responding to the write message before writing them to permanent storage. The server will delay the response to a sync message until after all blocks in earlier write messages have been written to permanent storage.

The goodbye message ends a session. There is no VtRgoodbye: upon receiving the VtTgoodbye message, the server terminates up the connection.

venti(1), venti(2), venti(8)
Sean Quinlan and Sean Dorward, ``Venti: a new approach to archival storage'', Usenix Conference on File and Storage Technologies , 2002.
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