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VENTI(8)							      VENTI(8)

       venti - archival storage server

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

       Venti  is a SHA1-addressed archival storage server.  See venti(6) 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.

       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
       index,  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
       employed to access the data log efficiently and can be rebuilt if  lost
       or damaged.

       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
       index  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
       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 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 × nblock ≤ 0.7 × 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 venti-fmt(8))  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
       venti(6), 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.

	      The current integer value of variable.  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
	      index 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.

	      Set variable to value.

	      A	 PNG  image  graphing  the named run-time statistic over time.
	      The details  of  names  and  parameters  are  undocumented;  see
	      httpd.c in the venti sources.

       /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
	      request 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 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 # 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
       appended	 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 tcp!*!http)

	      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.
	      Implies -s.

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

       -m     Allocate free-memory-percent percent of the available free  RAM,
	      and  partition  it  per the guidelines in the Memory subsection.
	      This percentage should be large enough  to  include  the	entire
	      bloom  filter.   This  overrides all other memory sizing parame‐
	      ters, including those on the command line and in the  configura‐
	      tion file.  25% is a reasonable choice.

       -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

       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


       venti(1), venti(2), venti(6), venti-backup(8), venti-fmt(8)
       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.

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                          \ \ / /   \ \ / /   \ \ / /   
                           \   /     \   /     \   /    
                            \_/       \_/       \_/ 
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