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VENTI(8)		    System Manager's Manual		      VENTI(8)

       venti - archival	storage	server

       venti/venti  [ -Ldrs ] [	-a address ] [ -B blockcachesize ] [ -c	config
	 ] [ -C	lumpcachesize ]	[ -h httpaddress ] [ -I	indexcachesize ] [  -W
	 webroot ]

       Venti  is a SHA1-addressed archival storage server.  See	for a full in-
       troduction to the system.  This page documents the structure and	opera-
       tion 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.

       The venti server	maintains three	disk structures, typically  stored  on
       raw  disk partitions: the append-only data log, which holds, in sequen-
       tial order, the contents	of every block written to the server; the  in-
       dex,  which  helps  locate a block in the data log given	its score; and
       optionally the 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 em-
       ployed to access	the data log efficiently and can be rebuilt if lost or

       The  data log is	logically split	into sections called 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
       arena partitions.  Arena	partitions are filled in the  order  given  in
       the configuration.

       The  index  is  logically split into block-sized	pieces called buckets,
       each of which is	responsible for	a particular range of scores.  An  in-
       dex  may	 be  split across many disks, each storing many	buckets.  Such
       disks are called	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 ac-
       tive 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 not already
       stored in the index.  The bloom filter starts out zeroed.   Each	 score
       recorded	 in  the bloom filter is hashed	to choose nhash	bits to	set in
       the bloom filter.  A score is definitely	not stored in the index	of any
       of  its	nhash bits are not set.	 The bloom filter thus has two parame-
       ters: nhash (maximum 32)	and the	total bitmap size (maximum 512MB,  232

       The  bloom  filter  should  be sized so that nhash x nblock <= 0.7 x b,
       where nblock is the expected number of blocks stored on the server  and
       b  is  the  bitmap  size	in bits.  The false positive rate of the bloom
       filter when sized this way is approximately 2-nblock.  Nhash less  than
       10  are	not very useful; nhash greater than 24 are probably a waste of
       memory.	Fmtbloom (see can be given either nhash	or  nblock;  if	 given
       nblock, it will derive an appropriate nhash.

       Venti  can  make	 effective  use	of large amounts of memory for various

       The lump	cache holds recently-accessed venti  data  blocks,  which  the
       server  refers  to as 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

       The block cache holds recently-accessed disk blocks from	the arena par-
       titions.	  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.

       The 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).

       The bloom filter	requires memory	equal to its size  on  disk,  as  dis-
       cussed 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.

       The  venti  server  announces two network services, one (conventionally
       TCP port	venti, 17034) serving the venti	protocol as described  in  and
       one serving HTTP	(conventionally	TCP port http, 80).

       The  venti  web server provides the following URLs for accessing	status

       /index A	summary	of the usage of	the arenas and index sections.

	      An XML version of	/index.

	      Brief storage totals.

       /set   Disable the values of all	variables.  Variables  are:  compress,
	      whether  or  not	to  compress  blocks (for debugging); logging,
	      whether to write entries to the debugging	logs;  stats,  whether
	      to  collect  run-time  statistics;  icachesleeptime, the time in
	      milliseconds between successive updates of megabytes of the  in-
	      dex  cache;  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	 vari-
	      ables exist only for debugging and performance measurement.

	      Show the current setting of variable.

	      Set variable to value.

	      A	 PNG image graphing the	run-time statistic over	time.  The de-
	      tails of names and parameters are	mostly undocumented;  see  the
	      graphname	 array in httpd.c in the venti code for	a list of pos-
	      sible statistics.	The type of graph defaults  to	raw,  see  the
	      xgraph  function for a list of types. Possible param include the
	      timeframe	(t0 and	t1) , the y limits (min	and max)

       /log   A	list of	all debugging logs present in the server's memory.

	      The contents of the debugging log	with the given name.

	      Force venti to begin flushing the	index cache to disk.  The  re-
	      quest response will not be sent until the	flush has completed.

	      Force  venti  to	begin  flushing	the arena block	cache to disk.
	      The request response will	not be sent until the flush  has  com-

       Requests	 for other files are served by consulting a directory named in
       the configuration file (see webroot below).

   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 for-
       matted with fmtarenas or	fmtisect (see In particular, only the configu-
       ration 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 # are comments.

       index name
	      Names the	index for the system.

       arenas file
	      File is an arena partition, formatted using fmtarenas.

       isect file
	      File is an index section,	formatted using	fmtisect.

       bloom file
	      File is a	bloom filter, formatted	using fmtbloom.

       After formatting	a venti	system using fmtindex, the order of arenas and
       index sections should not be changed.  Additional  arenas  can  be  ap-
       pended  to  the	configuration; run fmtindex with the -a	flag to	update
       the index.

       The configuration file also  holds  configuration  parameters  for  the
       venti server itself.  These are:

       mem size
	      lump cache size

       bcmem size
	      block cache size

       icmem size
	      index cache size

       addr netaddr
	      network address to announce venti	service	(default tcp!*!venti)

       httpaddr	netaddr
	      network  address	to  announce  HTTP  service (default is	not to
	      start the	service)

	      queue writes in memory (default is not to	queue)

       webroot dir
	      directory	tree containing	files for venti's internal HTTP	server
	      to consult for unrecognized URLs

       The units for the various cache sizes above can be specified by append-
       ing a or	(case-insensitive) to indicate kilobytes, megabytes, or	 giga-
       bytes respectively.

       The  file  name	in  the	 configuration	lines 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.

   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:

       -c config
	      The server configuration file (default venti.conf)

       -d     Produce  various	debugging  information on standard error.  Im-
	      plies -s.

       -L     Enable logging.  By default all logging  is  disabled.   Logging
	      slows server operation considerably.

       -r     Allow only read access to	the venti data.

       -s     Do  not run in the background.  Normally,	the foreground process
	      will exit	once the Venti server is  initialized  and  ready  for

       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


       Sean Quinlan and	Sean Dorward, ``Venti:	a  new	approach  to  archival
       storage'', Usenix Conference on File and	Storage	Technologies , 2002.

       Setting up a venti server is too	complicated.

       Venti should not	require	the user to decide how to partition its	memory

       Users of	shells other than will not be able to use  the	program	 names
       shown.	One  solution is to define V=$PLAN9/bin/venti and then substi-
       tute $V/	for venti/ in the paths	above.



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