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venti \- archival storage server
.in +0.25i
.ti -0.25i
.B venti/venti
.B -Ldrs
.B -a
.I address
.B -B
.I blockcachesize
.B -c
.I config
.B -C
.I lumpcachesize
.B -h
.I httpaddress
.B -I
.I indexcachesize
.B -W
.I webroot
.I Venti
is a SHA1-addressed archival storage server.
.IR venti (7)
for a full introduction to the system.
This page documents the structure and operation of the server.
A venti server requires multiple disks or disk partitions,
each of which must be properly formatted before the server
can be run.
.SS Disk
The venti server maintains three disk structures, typically
stored on raw disk partitions:
the append-only
.IR "data log" ,
which holds, in sequential order,
the contents of every block written to the server;
.IR index ,
which helps locate a block in the data log given its score;
and optionally the
.IR "bloom filter" ,
a concise summary of which scores are present in the index.
The data log is the primary storage.
To improve the robustness, it should be stored on
a device that provides RAID functionality.
The index and the bloom filter are optimizations
employed to access the data log efficiently and can be rebuilt
if lost or damaged.
The data log is logically split into sections called
.IR arenas ,
typically sized for easy offline backup
(e.g., 500MB).
A data log may comprise many disks, each storing
one or more arenas.
Such disks are called
.IR "arena partitions" .
Arena partitions are filled in the order given in the configuration.
The index is logically split into block-sized pieces called
.IR buckets ,
each of which is responsible for a particular range of scores.
An index may be split across many disks, each storing many buckets.
Such disks are called
.IR "index sections" .
The index must be sized so that no bucket is full.
When a bucket fills, the server must be shut down and
the index made larger.
Since scores appear random, each bucket will contain
approximately the same number of entries.
Index entries are 40 bytes long. Assuming that a typical block
being written to the server is 8192 bytes and compresses to 4096
bytes, the active index is expected to be about 1% of
the active data log.
Storing smaller blocks increases the relative index footprint;
storing larger blocks decreases it.
To allow variation in both block size and the random distribution
of scores to buckets, the suggested index size is 5% of
the active data log.
The (optional) bloom filter is a large bitmap that is stored on disk but
also kept completely in memory while the venti server runs.
It helps the venti server efficiently detect scores that are
.I not
already stored in the index.
The bloom filter starts out zeroed.
Each score recorded in the bloom filter is hashed to choose
.I nhash
bits to set in the bloom filter.
A score is definitely not stored in the index of any of its
.I nhash
bits are not set.
The bloom filter thus has two parameters:
.I nhash
(maximum 32)
and the total bitmap size
(maximum 512MB, 2\s-2\u32\d\s+2 bits).
The bloom filter should be sized so that
.I nhash
.I nblock
0.7 \(mu
.IR b ,
.I nblock
is the expected number of blocks stored on the server
.I b
is the bitmap size in bits.
The false positive rate of the bloom filter when sized
this way is approximately 2\s-2\u\-\fInblock\fR\d\s+2.
.I Nhash
less than 10 are not very useful;
.I nhash
greater than 24 are probably a waste of memory.
.I Fmtbloom
.IR venti-fmt (8))
can be given either
.I nhash
.IR nblock ;
if given
.IR nblock ,
it will derive an appropriate
.IR nhash .
.SS Memory
Venti can make effective use of large amounts of memory
for various caches.
.I "lump cache
holds recently-accessed venti data blocks, which the server refers to as
.IR lumps .
The lump cache should be at least 1MB but can profitably be much larger.
The lump cache can be thought of as the level-1 cache:
read requests handled by the lump cache can
be served instantly.
.I "block cache
holds recently-accessed
.I disk
blocks from the arena partitions.
The block cache needs to be able to simultaneously hold two blocks
from each arena plus four blocks for the currently-filling arena.
The block cache can be thought of as the level-2 cache:
read requests handled by the block cache are slower than those
handled by the lump cache, since the lump data must be extracted
from the raw disk blocks and possibly decompressed, but no
disk accesses are necessary.
.I "index cache
holds recently-accessed or prefetched
index entries.
The index cache needs to be able to hold index entries
for three or four arenas, at least, in order for prefetching
to work properly. Each index entry is 50 bytes.
Assuming 500MB arenas of
128,000 blocks that are 4096 bytes each after compression,
the minimum index cache size is about 6MB.
The index cache can be thought of as the level-3 cache:
read requests handled by the index cache must still go
to disk to fetch the arena blocks, but the costly random
access to the index is avoided.
The size of the index cache determines how long venti
can sustain its `burst' write throughput, during which time
the only disk accesses on the critical path
are sequential writes to the arena partitions.
For example, if you want to be able to sustain 10MB/s
for an hour, you need enough index cache to hold entries
for 36GB of blocks. Assuming 8192-byte blocks,
you need room for almost five million index entries.
Since index entries are 50 bytes each, you need 250MB
of index cache.
If the background index update process can make a single
pass through the index in an hour, which is possible,
then you can sustain the 10MB/s indefinitely (at least until
the arenas are all filled).
.I "bloom filter
requires memory equal to its size on disk,
as discussed above.
A reasonable starting allocation is to
divide memory equally (in thirds) between
the bloom filter, the index cache, and the lump and block caches;
the third of memory allocated to the lump and block caches
should be split unevenly, with more (say, two thirds)
going to the block cache.
.SS Network
The venti server announces two network services, one
(conventionally TCP port
.BR venti ,
17034) serving
the venti protocol as described in
.IR venti (7),
and one serving HTTP
(conventionally TCP port
.BR http ,
The venti web server provides the following
URLs for accessing status information:
.TF "\fL/storage"
.B /index
A summary of the usage of the arenas and index sections.
.B /xindex
An XML version of
.BR /index .
.B /storage
Brief storage totals.
.BI /set
Disable the values of all variables.
Variables are:
.BR compress ,
whether or not to compress blocks
(for debugging);
.BR logging ,
whether to write entries to the debugging logs;
.BR stats ,
whether to collect run-time statistics;
.BR icachesleeptime ,
the time in milliseconds between successive updates
of megabytes of the index cache;
.BR arenasumsleeptime ,
the time in milliseconds between reads while
checksumming an arena in the background.
The two sleep times should be (but are not) managed by venti;
they exist to provide more experience with their effects.
The other variables exist only for debugging and
performance measurement.
.BI /set?name= variable
Show the current setting of
.IR variable .
.BI /set?name= variable &value= value
.I variable
.IR value .
.BI /graph/ name / param / param / \fR...
A PNG image graphing the named run-time statistic over time.
The details of names and parameters are undocumented;
.B httpd.c
in the venti sources.
.B /log
A list of all debugging logs present in the server's memory.
.BI /log/ name
The contents of the debugging log with the given
.IR name .
.B /flushicache
Force venti to begin flushing the index cache to disk.
The request response will not be sent until the flush
has completed.
.B /flushdcache
Force venti to begin flushing the arena block cache to disk.
The request response will not be sent until the flush
has completed.
Requests for other files are served by consulting a
directory named in the configuration file
.B webroot
.SS Configuration File
A venti configuration file
enumerates the various index sections and
arenas that constitute a venti system.
The components are indicated by the name of the file, typically
a disk partition, in which they reside. The configuration
file is the only location that file names are used. Internally,
venti uses the names assigned when the components were formatted
.I fmtarenas
.I fmtisect
.IR venti-fmt (8)).
In particular, only the configuration needs to be
changed if a component is moved to a different file.
The configuration file consists of lines in the form described below.
Lines starting with
.B #
are comments.
.TF "\fLindex\fI name "
.BI index " name
Names the index for the system.
.BI arenas " file
.I File
is an arena partition, formatted using
.IR fmtarenas .
.BI isect " file
.I File
is an index section, formatted using
.IR fmtisect .
.BI bloom " file
.I File
is a bloom filter, formatted using
.IR fmtbloom .
After formatting a venti system using
.IR fmtindex ,
the order of arenas and index sections should not be changed.
Additional arenas can be appended to the configuration;
.I fmtindex
with the
.B -a
flag to update the index.
The configuration file also holds configuration parameters
for the venti server itself.
These are:
.TF "\fLhttpaddr\fI netaddr "
.BI mem " size
lump cache size
.BI bcmem " size
block cache size
.BI icmem " size
index cache size
.BI addr " netaddr
network address to announce venti service
.BR tcp!*!venti )
.BI httpaddr " netaddr
network address to announce HTTP service
.BR tcp!*!http )
.B queuewrites
queue writes in memory
(default is not to queue)
.BI webroot " dir
directory tree containing files for
.IR venti 's
internal HTTP server to consult for unrecognized URLs
The units for the various cache sizes above can be specified by appending a
.LR k ,
.LR m ,
.LR g
to indicate kilobytes, megabytes, or gigabytes respectively.
.I file
name in the configuration lines above can be of the form
.IB file : lo - hi
to specify a range of the file.
.I Lo
.I hi
are specified in bytes but can have the usual
.BI k ,
.BI m ,
.B g
.I lo
.I hi
may be omitted.
This notation eliminates the need to
partition raw disks on non-Plan 9 systems.
.SS Command Line
Many of the options to Venti duplicate parameters that
can be specified in the configuration file.
The command line options override those found in a
configuration file.
Additional options are:
.TF "\fL-c\fI config"
.BI -c " config
The server configuration file
.BR venti.conf )
.B -d
Produce various debugging information on standard error.
.BR -s .
.B -L
Enable logging. By default all logging is disabled.
Logging slows server operation considerably.
.B -r
Allow only read access to the venti data.
.B -s
Do not run in the background.
the foreground process will exit once the Venti server
is initialized and ready for connections.
A simple configuration:
% cat venti.conf
index main
isect /tmp/disks/isect0
isect /tmp/disks/isect1
arenas /tmp/disks/arenas
bloom /tmp/disks/bloom
mem 10M
bcmem 20M
icmem 30M
Format the index sections, the arena partition,
the bloom filter, and
finally the main index:
% venti/fmtisect isect0. /tmp/disks/isect0
% venti/fmtisect isect1. /tmp/disks/isect1
% venti/fmtarenas arenas0. /tmp/disks/arenas &
% venti/fmtbloom /tmp/disks/bloom &
% wait
% venti/fmtindex venti.conf
Start the server and check the storage statistics:
% venti/venti
% hget http://$sysname/storage
.B \*9/src/cmd/venti/srv
.IR venti (1),
.IR venti (3),
.IR venti (7),
.IR venti-backup (8)
.IR venti-fmt (8)
Sean Quinlan and Sean Dorward,
``Venti: a new approach to archival storage'',
.I "Usenix Conference on File and Storage Technologies" ,
Setting up a venti server is too complicated.
Venti should not require the user to decide how to
partition its memory usage.
Users of shells other than
.IR rc (1)
will not be able to use the program names shown.
One solution is to define
.B "V=$PLAN9/bin/venti"
and then substitute
.B $V/
.B venti/
in the paths above.