512 lines
12 KiB
Text
512 lines
12 KiB
Text
.TH VENTI 8
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.SH NAME
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venti \- archival storage server
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.SH SYNOPSIS
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.in +0.25i
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.ti -0.25i
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.B venti/venti
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[
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.B -Ldrs
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]
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[
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.B -a
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.I address
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]
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[
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.B -B
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.I blockcachesize
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]
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[
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.B -c
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.I config
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]
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[
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.B -C
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.I lumpcachesize
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]
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[
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.B -h
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.I httpaddress
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]
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[
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.B -I
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.I indexcachesize
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]
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[
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.B -m
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.I free-memory%
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]
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[
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.B -W
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.I webroot
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]
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.SH DESCRIPTION
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.I Venti
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is a SHA1-addressed archival storage server.
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See
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.IR venti (6)
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for a full introduction to the system.
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This page documents the structure and operation of the server.
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.PP
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A venti server requires multiple disks or disk partitions,
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each of which must be properly formatted before the server
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can be run.
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.SS Disk
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The venti server maintains three disk structures, typically
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stored on raw disk partitions:
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the append-only
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.IR "data log" ,
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which holds, in sequential order,
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the contents of every block written to the server;
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the
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.IR index ,
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which helps locate a block in the data log given its score;
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and optionally the
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.IR "bloom filter" ,
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a concise summary of which scores are present in the index.
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The data log is the primary storage.
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To improve the robustness, it should be stored on
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a device that provides RAID functionality.
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The index and the bloom filter are optimizations
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employed to access the data log efficiently and can be rebuilt
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if lost or damaged.
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.PP
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The data log is logically split into sections called
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.IR arenas ,
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typically sized for easy offline backup
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(e.g., 500MB).
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A data log may comprise many disks, each storing
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one or more arenas.
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Such disks are called
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.IR "arena partitions" .
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Arena partitions are filled in the order given in the configuration.
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.PP
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The index is logically split into block-sized pieces called
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.IR buckets ,
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each of which is responsible for a particular range of scores.
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An index may be split across many disks, each storing many buckets.
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Such disks are called
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.IR "index sections" .
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.PP
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The index must be sized so that no bucket is full.
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When a bucket fills, the server must be shut down and
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the index made larger.
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Since scores appear random, each bucket will contain
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approximately the same number of entries.
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Index entries are 40 bytes long. Assuming that a typical block
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being written to the server is 8192 bytes and compresses to 4096
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bytes, the active index is expected to be about 1% of
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the active data log.
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Storing smaller blocks increases the relative index footprint;
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storing larger blocks decreases it.
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To allow variation in both block size and the random distribution
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of scores to buckets, the suggested index size is 5% of
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the active data log.
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.PP
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The (optional) bloom filter is a large bitmap that is stored on disk but
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also kept completely in memory while the venti server runs.
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It helps the venti server efficiently detect scores that are
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.I not
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already stored in the index.
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The bloom filter starts out zeroed.
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Each score recorded in the bloom filter is hashed to choose
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.I nhash
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bits to set in the bloom filter.
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A score is definitely not stored in the index of any of its
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.I nhash
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bits are not set.
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The bloom filter thus has two parameters:
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.I nhash
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(maximum 32)
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and the total bitmap size
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(maximum 512MB, 2\s-2\u32\d\s+2 bits).
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.PP
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The bloom filter should be sized so that
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.I nhash
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\(mu
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.I nblock
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\(<=
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0.7 \(mu
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.IR b ,
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where
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.I nblock
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is the expected number of blocks stored on the server
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and
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.I b
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is the bitmap size in bits.
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The false positive rate of the bloom filter when sized
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this way is approximately 2\s-2\u\-\fInblock\fR\d\s+2.
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.I Nhash
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less than 10 are not very useful;
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.I nhash
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greater than 24 are probably a waste of memory.
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.I Fmtbloom
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(see
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.IR venti-fmt (8))
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can be given either
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.I nhash
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or
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.IR nblock ;
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if given
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.IR nblock ,
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it will derive an appropriate
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.IR nhash .
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.SS Memory
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Venti can make effective use of large amounts of memory
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for various caches.
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.PP
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The
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.I "lump cache
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holds recently-accessed venti data blocks, which the server refers to as
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.IR lumps .
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The lump cache should be at least 1MB but can profitably be much larger.
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The lump cache can be thought of as the level-1 cache:
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read requests handled by the lump cache can
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be served instantly.
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.PP
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The
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.I "block cache
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holds recently-accessed
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.I disk
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blocks from the arena partitions.
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The block cache needs to be able to simultaneously hold two blocks
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from each arena plus four blocks for the currently-filling arena.
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The block cache can be thought of as the level-2 cache:
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read requests handled by the block cache are slower than those
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handled by the lump cache, since the lump data must be extracted
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from the raw disk blocks and possibly decompressed, but no
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disk accesses are necessary.
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.PP
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The
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.I "index cache
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holds recently-accessed or prefetched
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index entries.
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The index cache needs to be able to hold index entries
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for three or four arenas, at least, in order for prefetching
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to work properly. Each index entry is 50 bytes.
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Assuming 500MB arenas of
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128,000 blocks that are 4096 bytes each after compression,
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the minimum index cache size is about 6MB.
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The index cache can be thought of as the level-3 cache:
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read requests handled by the index cache must still go
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to disk to fetch the arena blocks, but the costly random
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access to the index is avoided.
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.PP
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The size of the index cache determines how long venti
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can sustain its `burst' write throughput, during which time
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the only disk accesses on the critical path
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are sequential writes to the arena partitions.
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For example, if you want to be able to sustain 10MB/s
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for an hour, you need enough index cache to hold entries
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for 36GB of blocks. Assuming 8192-byte blocks,
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you need room for almost five million index entries.
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Since index entries are 50 bytes each, you need 250MB
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of index cache.
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If the background index update process can make a single
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pass through the index in an hour, which is possible,
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then you can sustain the 10MB/s indefinitely (at least until
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the arenas are all filled).
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.PP
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The
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.I "bloom filter
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requires memory equal to its size on disk,
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as discussed above.
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.PP
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A reasonable starting allocation is to
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divide memory equally (in thirds) between
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the bloom filter, the index cache, and the lump and block caches;
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the third of memory allocated to the lump and block caches
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should be split unevenly, with more (say, two thirds)
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going to the block cache.
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.SS Network
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The venti server announces two network services, one
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(conventionally TCP port
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.BR venti ,
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17034) serving
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the venti protocol as described in
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.IR venti (6),
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and one serving HTTP
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(conventionally TCP port
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.BR http ,
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80).
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.PP
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The venti web server provides the following
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URLs for accessing status information:
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.TF "\fL/storage"
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.PD
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.TP
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.B /index
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A summary of the usage of the arenas and index sections.
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.TP
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.B /xindex
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An XML version of
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.BR /index .
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.TP
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.B /storage
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Brief storage totals.
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.TP
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.BI /set/ variable
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The current integer value of
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.IR variable .
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Variables are:
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.BR compress ,
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whether or not to compress blocks
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(for debugging);
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.BR logging ,
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whether to write entries to the debugging logs;
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.BR stats ,
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whether to collect run-time statistics;
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.BR icachesleeptime ,
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the time in milliseconds between successive updates
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of megabytes of the index cache;
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.BR arenasumsleeptime ,
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the time in milliseconds between reads while
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checksumming an arena in the background.
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The two sleep times should be (but are not) managed by venti;
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they exist to provide more experience with their effects.
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The other variables exist only for debugging and
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performance measurement.
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.TP
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.BI /set/ variable / value
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Set
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.I variable
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to
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.IR value .
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.TP
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.BI /graph/ name / param / param / \fR...
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A PNG image graphing the named run-time statistic over time.
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The details of names and parameters are undocumented;
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see
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.B httpd.c
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in the venti sources.
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.TP
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.B /log
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A list of all debugging logs present in the server's memory.
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.TP
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.BI /log/ name
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The contents of the debugging log with the given
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.IR name .
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.TP
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.B /flushicache
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Force venti to begin flushing the index cache to disk.
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The request response will not be sent until the flush
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has completed.
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.TP
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.B /flushdcache
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Force venti to begin flushing the arena block cache to disk.
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The request response will not be sent until the flush
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has completed.
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.PD
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.PP
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Requests for other files are served by consulting a
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directory named in the configuration file
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(see
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.B webroot
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below).
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.SS Configuration File
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A venti configuration file
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enumerates the various index sections and
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arenas that constitute a venti system.
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The components are indicated by the name of the file, typically
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a disk partition, in which they reside. The configuration
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file is the only location that file names are used. Internally,
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venti uses the names assigned when the components were formatted
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with
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.I fmtarenas
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or
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.I fmtisect
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(see
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.IR venti-fmt (8)).
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In particular, only the configuration needs to be
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changed if a component is moved to a different file.
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.PP
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The configuration file consists of lines in the form described below.
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Comments start with
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.B #
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and continue until the end of the line.
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.TF "\fLindex\fI name "
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.PD
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.TP
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.BI index " name
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Names the index for the system.
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.TP
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.BI arenas " file
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.I File
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is an arena partition, formatted using
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.IR fmtarenas .
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.TP
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.BI isect " file
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.I File
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is an index section, formatted using
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.IR fmtisect .
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.TP
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.BI bloom " file
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.I File
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is a bloom filter, formatted using
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.IR fmtbloom .
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.PD
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.PP
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After formatting a venti system using
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.IR fmtindex ,
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the order of arenas and index sections should not be changed.
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Additional arenas can be appended to the configuration;
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run
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.I fmtindex
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with the
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.B -a
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flag to update the index.
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.PP
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The configuration file also holds configuration parameters
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for the venti server itself.
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These are:
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.TF "\fLhttpaddr\fI netaddr "
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.TP
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.BI mem " size
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lump cache size
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.TP
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.BI bcmem " size
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block cache size
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.TP
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.BI icmem " size
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index cache size
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.TP
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.BI addr " netaddr
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network address to announce venti service
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(default
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.BR tcp!*!venti )
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.TP
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.BI httpaddr " netaddr
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network address to announce HTTP service
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(default
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.BR tcp!*!http )
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.TP
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.B queuewrites
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queue writes in memory
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(default is not to queue)
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.TP
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.BI webroot " dir
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directory tree containing files for
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.IR venti 's
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internal HTTP server to consult for unrecognized URLs
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.PD
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.PP
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The units for the various cache sizes above can be specified by appending a
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.LR k ,
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.LR m ,
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or
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.LR g
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(case-insensitive)
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to indicate kilobytes, megabytes, or gigabytes respectively.
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.PP
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The
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.I file
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name in the configuration lines above can be of the form
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.IB file : lo - hi
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to specify a range of the file.
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.I Lo
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and
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.I hi
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are specified in bytes but can have the usual
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.BI k ,
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.BI m ,
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or
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.B g
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suffixes.
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Either
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.I lo
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or
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.I hi
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may be omitted.
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This notation eliminates the need to
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partition raw disks on non-Plan 9 systems.
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.SS Command Line
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Many of the options to Venti duplicate parameters that
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can be specified in the configuration file.
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The command line options override those found in a
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configuration file.
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Additional options are:
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.TF "\fL-c\fI config"
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.PD
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.TP
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.BI -c " config
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The server configuration file
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(default
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.BR venti.conf )
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.TP
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.B -d
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Produce various debugging information on standard error.
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Implies
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.BR -s .
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.TP
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.B -L
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Enable logging. By default all logging is disabled.
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Logging slows server operation considerably.
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.TP
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.B -m
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Allocate
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.I free-memory%
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percent of the available free RAM, and partition it
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per the guidelines in the
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.B Memory
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subsection.
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This percentage should be large enough to include the entire bloom filter.
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This overrides all other memory sizing parameters,
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including those on the command line and in the configuration file.
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.TP
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.B -r
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Allow only read access to the venti data.
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.TP
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.B -s
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Do not run in the background.
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Normally,
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the foreground process will exit once the Venti server
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is initialized and ready for connections.
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.PD
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.SH EXAMPLE
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A simple configuration:
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.IP
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.EX
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% cat venti.conf
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index main
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isect /tmp/disks/isect0
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isect /tmp/disks/isect1
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arenas /tmp/disks/arenas
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bloom /tmp/disks/bloom
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%
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.EE
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.PP
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Format the index sections, the arena partition,
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the bloom filter, and
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finally the main index:
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.IP
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.EX
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% venti/fmtisect isect0. /tmp/disks/isect0
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% venti/fmtisect isect1. /tmp/disks/isect1
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% venti/fmtarenas arenas0. /tmp/disks/arenas &
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% venti/fmtbloom /tmp/disks/bloom &
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% wait
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% venti/fmtindex venti.conf
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%
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.EE
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.PP
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Start the server and check the storage statistics:
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.IP
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.EX
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% venti/venti
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% hget http://$sysname/storage
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.EE
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.SH SOURCE
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.B /sys/src/cmd/venti/srv
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.SH "SEE ALSO"
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.IR venti (1),
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.IR venti (2),
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.IR venti (6),
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.IR venti-backup (8),
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.IR venti-fmt (8)
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.br
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Sean Quinlan and Sean Dorward,
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``Venti: a new approach to archival storage'',
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.I "Usenix Conference on File and Storage Technologies" ,
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2002.
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.SH BUGS
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Setting up a venti server is too complicated.
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