VENTI-FMT(8)

NAME
buildindex, checkarenas, checkindex, conf, fmtarenas, fmtbloom, fmtindex, fmtisect, syncindex - prepare and maintain a venti server

SYNOPSIS

venti/fmtarenas [ -Z ] [ -a arenasize ] [ -b blocksize ] name file

venti/fmtisect [ -1Z ] [ -b blocksize ] name file

venti/fmtbloom [ -n nblocks | -N nhash ] [ -s size ] file

venti/fmtindex [ -a ] venti.conf

venti/conf [ -w ] partition [ configfile ]

venti/buildindex [ -B blockcachesize ] [ -Z ] venti.conf tmp

venti/checkindex [ -f ] [ -B blockcachesize ] venti.conf tmp

venti/checkarenas [ -afv ] file

venti/copy [ -f ] src dst score [ type ]

DESCRIPTION
These commands aid in the setup, maintenance, and debugging of venti servers. See venti(7) for an overview of the venti system and venti(8) for an overview of the data structures used by the venti server.

Note that the units for the various sizes in the following commands can be specified by appending or to indicate kilobytes, megabytes, or gigabytes respectively.

  Formatting
To prepare a server for its initial use, the arena partitions and the index sections must be formatted individually, with fmtarenas and fmtisect. Then the collection of index sections must be combined into a venti index with fmtindex.

Fmtarenas formats the given file, typically a disk partition, into an arena partition. The arenas in the partition are given names of the form name%d, where %d is replaced with a sequential number starting at 0.

Options to fmtarenas are:

-a arenasize
    The arenas are of arenasize bytes. The default is 512M, which was selected to provide a balance between the number of arenas and the ability to copy an arena to external media such as recordable CDs and tapes.

-b blocksize
    The size, in bytes, for read and write operations to the file. The size is recorded in the file, and is used by applications that access the arenas. The default is 8k.

-4  Create a `version 4' arena partition for backwards compatibility with old servers. The default is version 5, used by the current venti server.

-Z  Do not zero the data sections of the arenas. Using this option reduces the formatting time but should only be used when it is known that the file was already zeroed. (Version 4 only; version 5 sections are not and do not need to be zeroed.)

Fmtisect formats the given file, typically a disk partition, as a venti index section with the specified name. Each of the index sections in a venti configuration must have a unique name.

Options to fmtisect are:

-b bucketsize
    The size of an index bucket, in bytes. All the index sections within a index must have the same bucket size. The default is 8k.

-1  Create a `version 1' index section for backwards compatibility with old servers. The default is version 2, used by the current venti server.

-Z  Do not zero the index. Using this option reduces the formatting time but should only be used when it is known that the file was already zeroed. (Version 1 only; version 2 sections are not and do not need to be zeroed.)

Fmtbloom formats the given file as a bloom filter (see venti(7)). The options are:

-n nblock | -N nhash
    The number of blocks expected to be indexed by the filter or the number of hash functions to use. If the -n option is given, it is used, along with the total size of the filter, to compute an appropriate nhash.

-s size
    The size of the bloom filter. The default is the total size of the file. In either case, size is rounded down to a power of two.

The file argument in the commands above can be of the form file:lo-hi to specify a range of the file. Lo and hi are specified in bytes but can have the usual k, m, or g suffixes. Either lo or hi may be omitted. This notation eliminates the need to partition raw disks on non-Plan 9 systems.

Fmtindex reads the configuration file venti.conf and initializes the index sections to form a usable index structure. The arena files and index sections must have previously been formatted using fmtarenas and fmtisect respectively.

The function of a venti index is to map a SHA1 fingerprint to a location in the data section of one of the arenas. The index is composed of blocks, each of which contains the mapping for a fixed range of possible fingerprint values. Fmtindex determines the mapping between SHA1 values and the blocks of the collection of index sections. Once this mapping has been determined, it cannot be changed without rebuilding the index. The basic assumption in the current implementation is that the index structure is sufficiently empty that individual blocks of the index will rarely overflow. The total size of the index should be about 2% to 10% of the total size of the arenas, but the exact percentage depends both on the index block size and the compressed size of blocks stored. See the discussion in venti(8) for more.

Fmtindex also computes a mapping between a linear address space and the data section of the collection of arenas. The -a option can be used to add additional arenas to an index. To use this feature, add the new arenas to venti.conf after the existing arenas and then run fmtindex -a.

A copy of the above mappings is stored in the header for each of the index sections. These copies enable buildindex to restore a single index section without rebuilding the entire index.

To make it easier to bootstrap servers, the configuration file can be stored in otherwise empty space at the beginning of any venti partitions using conf. A partition so branded with a configuration file can be used in place of a configuration file when invoking any of the venti commands. By default, conf prints the configuration stored in partition. When invoked with the -w flag, conf reads a configuration file from configfile (or else standard input) and stores it in partition.

  Checking and Rebuilding

Buildindex populates the index for the Venti system described in venti.conf. The index must have previously been formatted using fmtindex. This command is typically used to build a new index for a Venti system when the old index becomes too small, or to rebuild an index after media failure. Small errors in an index can usually be fixed with checkindex.

The tmp file, usually a disk partition, must be large enough to store a copy of the index. This temporary space is used to perform a merge sort of index entries generated by reading the arenas.

Options to buildindex are:

-B blockcachesize
    The amount of memory, in bytes, to use for caching raw disk accesses while running buildindex. (This is not a property of the created index.) The default is 8k.

-Z  Do not zero the index. This option should only be used when it is known that the index was already zeroed. (Version 1 indexes only; see the discussion in fmtindex above.)

Checkindex examines the Venti index described in venti.conf. The program detects various error conditions including: blocks that are not indexed, index entries for blocks that do not exist, and duplicate index entries. If requested, an attempt can be made to fix errors that are found.

The tmp file, usually a disk partition, must be large enough to store a copy of the index. This temporary space is used to perform a merge sort of index entries generated by reading the arenas.

Options to checkindex are:

-B blockcachesize
    The amount of memory, in bytes, to use for caching raw disk accesses while running checkindex. The default is 8k.

-f  Attempt to fix any errors that are found.

Checkarenas examines the Venti arenas contained in the given file. The program detects various error conditions, and optionally attempts to fix any errors that are found.

Options to checkarenas are:

-a  For each arena, scan the entire data section. If this option is omitted, only the end section of the arena is examined.

-f  Attempt to fix any errors that are found.

-v  Increase the verbosity of output.

SOURCE
/usr/local/plan9/src/cmd/venti/srv

SEE ALSO
venti(7), venti(8)

BUGS
Buildindex should allow an individual index section to be rebuilt. The merge sort could be performed in the space used to store the index rather than requiring a temporary file.