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Venti is a SHA1-addressed archival storage server. See 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.
Disk
The venti server maintains three disk structures, typically stored
on raw disk partitions: the append-only data log, which holds,
in sequential 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 parameters: nhash
(maximum 32) and the total bitmap size (maximum 512MB, 232 bits).
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.
Memory
Venti can make effective use of large amounts of memory for various
caches.
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 instantly.
The block cache holds recently-accessed 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.
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
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.
Network
The venti server announces two network services, one (conventionally
TCP port venti, 17034) serving the venti protocol as described
in venti(7), and one serving HTTP (conventionally TCP port http,
80).
The venti web server provides the following URLs for accessing
status information:
/index A summary of the usage of the arenas and index sections.
/xindex An XML version of /index.
/storage 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 index cache;
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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.
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/set?name=variable
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Show the current setting of variable.
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/set?name=variable&value=value
/graph?arg=name[&arg2=name]&graph=type
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A PNG image graphing the run-time statistic over time. The details
of names and parameters are mostly undocumented; see the graphname
array in httpd.c in the venti code for a list of possible 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)
etc.
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/log A list of all debugging logs present in the server’s memory.
/log/nameThe contents of the debugging log with the given name.
/flushicache
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Force venti to begin flushing the index cache to disk. The request
response will not be sent until the flush has completed.
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/flushdcache
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Force venti to begin flushing the arena block cache to disk. The
request response will not be sent until the flush has completed.
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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
formatted 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 nameNames 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 netaddrnetwork address to announce HTTP service (default
is not to start the service)
queuewrites 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
appending a k, m, or g (case-insensitive) to indicate kilobytes,
megabytes, or gigabytes 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 configThe 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.
−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 connections.
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