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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), vacfs(4), and
vbackup(8) are more complex clients.
Venti(3) 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.
Scores
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,
while vbackup(8) uses prefixes corresponding to the file system
types: ext2:, ffs:, and so on.
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:
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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
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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(7)).
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:
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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]
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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(3)) 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.
Version 04 of the Venti protocol is similar to version 02 (described
above) but has two changes to accomodates larger payloads. First,
it replaces the leading 2-byte packet size with a 4-byte size.
Second, the count in the VtTread packet may be either 2 or 4 bytes;
the total packet length distinguishes the two cases.
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