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106
man/man8/venti-backup.8 Normal file
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.TH VENTI-BACKUP 8
.SH NAME
rdarena, wrarena \- copy arenas between venti servers
.SH SYNOPSIS
.PP
.B venti/rdarena
[
.B -v
]
.I arenapart
.I arenaname
.PP
.B venti/wrarena
[
.B -o
.I fileoffset
]
[
.B -h
.I host
]
.I arenafile
[
.I clumpoffset
]
.SH DESCRIPTION
.PP
.I Rdarena
extracts the named
.I arena
from the arena partition
.I arenapart
and writes this arena to standard output.
This command is typically used to back up an arena to external media.
The
.B -v
option generates more verbose output on standard error.
.PP
.I Wrarena
writes the blocks contained in the arena
.I arenafile
(typically, the output of
.IR rdarena )
to a Venti server.
It is typically used to reinitialize a Venti server from backups of the arenas.
For example,
.IP
.EX
venti/rdarena /dev/sdC0/arenas arena.0 >external.media
venti/wrarena -h venti2 external.media
.EE
.LP
writes the blocks contained in
.B arena.0
to the Venti server
.B venti2
(typically not the one using
.BR /dev/sdC0/arenas ).
.PP
The
.B -o
option specifies that the arena starts at byte
.I fileoffset
(default
.BR 0 )
in
.I arenafile .
This is useful for reading directly from
the Venti arena partition:
.IP
.EX
venti/wrarena -h venti2 -o 335872 /dev/sdC0/arenas
.EE
.LP
(In this example, 335872 is the offset shown in the Venti
server's index list (344064) minus one block (8192).
You will need to substitute your own arena offsets
and block size.)
.PP
Finally, the optional
.I offset
argument specifies that the writing should begin with the
clump starting at
.I offset
within the arena.
.I Wrarena
prints the offset it stopped at (because there were no more data blocks).
This could be used to incrementally back up a Venti server
to another Venti server:
.IP
.EX
last=`{cat last}
venti/wrarena -h venti2 -o 335872 /dev/sdC0/arenas $last >output
awk '/^end offset/ { print $3 }' offset >last
.EE
.LP
Of course, one would need to add wrapper code to keep track
of which arenas have been processed.
See
.B /sys/src/cmd/venti/backup.example
for a version that does this.
.SH SOURCE
.B \*9/src/cmd/venti/srv
.SH SEE ALSO
.IR venti (7),
.IR venti (8)

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.TH VENTI-FMT 8
.SH NAME
buildindex,
checkarenas,
checkindex,
conf,
fmtarenas,
fmtindex,
fmtisect,
syncindex \- prepare and maintain a venti server
.SH SYNOPSIS
.PP
.B venti/fmtarenas
[
.B -Z
]
[
.B -a
.I arenasize
]
[
.B -b
.I blocksize
]
.I name
.I file
.PP
.B venti/fmtisect
[
.B -1Z
]
[
.B -b
.I blocksize
]
.I name
.I file
.PP
.B venti/fmtindex
[
.B -a
]
.I venti.conf
.PP
.B venti/conf
[
.B -w
]
.I partition
[
.I configfile
]
.if t .sp 0.5
.PP
.B venti/buildindex
[
.B -B
.I blockcachesize
]
[
.B -Z
]
.I venti.conf
.I tmp
.PP
.B venti/checkindex
[
.B -f
]
[
.B -B
.I blockcachesize
]
.I venti.conf
.I tmp
.PP
.B venti/checkarenas
[
.B -afv
]
.I file
.PP
.B venti/copy
[
.B -f
]
.I src
.I dst
.I score
[
.I type
]
.SH DESCRIPTION
These commands aid in the setup, maintenance, and debugging of
venti servers.
See
.IR venti (7)
for an overview of the venti system and
.IR venti (8)
for an overview of the data structures used by the venti server.
.PP
Note that the units for the various sizes in the following
commands can be specified by appending
.LR k ,
.LR m ,
or
.LR g
to indicate kilobytes, megabytes, or gigabytes respectively.
.SS Formatting
To prepare a server for its initial use, the arena partitions and
the index sections must be formatted individually, with
.I fmtarenas
and
.IR fmtisect .
Then the
collection of index sections must be combined into a venti
index with
.IR fmtindex .
.PP
.I Fmtarenas
formats the given
.IR file ,
typically a disk partition, into an arena partition.
The arenas in the partition are given names of the form
.IR name%d ,
where
.I %d
is replaced with a sequential number starting at 0.
.PP
Options to
.I fmtarenas
are:
.TP
.BI -a " arenasize
The arenas are of
.I arenasize
bytes. The default is
.BR 512M ,
which was selected to provide a balance
between the number of arenas and the ability to copy an arena to external
media such as recordable CDs and tapes.
.TP
.BI -b " blocksize
The size, in bytes, for read and write operations to the file.
The size is recorded in the file, and is used by applications that access the arenas.
The default is
.BR 8k .
.TP
.B -4
Create a `version 4' arena partition for backwards compatibility with old servers.
The default is version 5, used by the current venti server.
.TP
.B -Z
Do not zero the data sections of the arenas.
Using this option reduces the formatting time
but should only be used when it is known that the file was already zeroed.
(Version 4 only; version 5 sections are not and do not need to be zeroed.)
.PD
.PP
.I Fmtisect
formats the given
.IR file ,
typically a disk partition, as a venti index section with the specified
.IR name .
Each of the index sections in a venti configuration must have a unique name.
.PP
Options to
.I fmtisect
are:
.TP
.BI -b " bucketsize
The size of an index bucket, in bytes.
All the index sections within a index must have the same bucket size.
The default is
.BR 8k .
.TP
.B -1
Create a `version 1' index section for backwards compatibility with old servers.
The default is version 2, used by the current venti server.
.TP
.B -Z
Do not zero the index.
Using this option reduces the formatting time
but should only be used when it is known that the file was already zeroed.
(Version 1 only; version 2 sections are not and do not need to be zeroed.)
.PD
.I Fmtindex
reads the configuration file
.I venti.conf
and initializes the index sections to form a usable index structure.
The arena files and index sections must have previously been formatted
using
.I fmtarenas
and
.I fmtisect
respectively.
.PP
The function of a venti index is to map a SHA1 fingerprint to a location
in the data section of one of the arenas. The index is composed of
blocks, each of which contains the mapping for a fixed range of possible
fingerprint values.
.I Fmtindex
determines the mapping between SHA1 values and the blocks
of the collection of index sections. Once this mapping has been determined,
it cannot be changed without rebuilding the index.
The basic assumption in the current implementation is that the index
structure is sufficiently empty that individual blocks of the index will rarely
overflow. The total size of the index should be about 2% to 10% of
the total size of the arenas, but the exact percentage depends both on the
index block size and the compressed size of blocks stored.
See the discussion in
.IR venti (8)
for more.
.PP
.I Fmtindex
also computes a mapping between a linear address space and
the data section of the collection of arenas. The
.B -a
option can be used to add additional arenas to an index.
To use this feature,
add the new arenas to
.I venti.conf
after the existing arenas and then run
.I fmtindex
.BR -a .
.PP
A copy of the above mappings is stored in the header for each of the index sections.
These copies enable
.I buildindex
to restore a single index section without rebuilding the entire index.
.PP
To make it easier to bootstrap servers, the configuration
file can be stored in otherwise empty space
at the beginning of any venti partitions using
.IR conf .
A partition so branded with a configuration file can
be used in place of a configuration file when invoking any
of the venti commands.
By default,
.I conf
prints the configuration stored in
.IR partition .
When invoked with the
.B -w
flag,
.I conf
reads a configuration file from
.I configfile
(or else standard input)
and stores it in
.IR partition .
.SS Checking and Rebuilding
.PP
.I Buildindex
populates the index for the Venti system described in
.IR venti.conf .
The index must have previously been formatted using
.IR fmtindex .
This command is typically used to build a new index for a Venti
system when the old index becomes too small, or to rebuild
an index after media failure.
Small errors in an index can usually be fixed with
.IR checkindex .
.PP
The
.I tmp
file, usually a disk partition, must be large enough to store a copy of the index.
This temporary space is used to perform a merge sort of index entries
generated by reading the arenas.
.PP
Options to
.I buildindex
are:
.TP
.BI -B " blockcachesize
The amount of memory, in bytes, to use for caching raw disk accesses while running
.IR buildindex .
(This is not a property of the created index.)
The default is 8k.
.TP
.B -Z
Do not zero the index.
This option should only be used when it is known that the index was already zeroed.
(Version 1 indexes only; see the discussion in
.I fmtindex
above.)
.PD
.PP
.I Checkindex
examines the Venti index described in
.IR venti.conf .
The program detects various error conditions including:
blocks that are not indexed, index entries for blocks that do not exist,
and duplicate index entries.
If requested, an attempt can be made to fix errors that are found.
.PP
The
.I tmp
file, usually a disk partition, must be large enough to store a copy of the index.
This temporary space is used to perform a merge sort of index entries
generated by reading the arenas.
.PP
Options to
.I checkindex
are:
.TP
.BI -B " blockcachesize
The amount of memory, in bytes, to use for caching raw disk accesses while running
.IR checkindex .
The default is 8k.
.TP
.B -f
Attempt to fix any errors that are found.
.PD
.PP
.I Checkarenas
examines the Venti arenas contained in the given
.IR file .
The program detects various error conditions, and optionally attempts
to fix any errors that are found.
.PP
Options to
.I checkarenas
are:
.TP
.B -a
For each arena, scan the entire data section.
If this option is omitted, only the end section of
the arena is examined.
.TP
.B -f
Attempt to fix any errors that are found.
.TP
.B -v
Increase the verbosity of output.
.PD
.SH SOURCE
.B \*9/src/cmd/venti/srv
.SH SEE ALSO
.IR venti (7),
.IR venti (8)
.SH BUGS
.I Buildindex
should allow an individual index section to be rebuilt.
The merge sort could be performed in the space used to store the
index rather than requiring a temporary file.

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.TH VENTI 8
.SH NAME
venti.conf \- venti configuration
.SH DESCRIPTION
Venti is a SHA1-addressed archival storage server.
See
.IR venti (7)
for a full introduction to the system.
This page documents the structure and operation of the server.
.PP
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;
the
.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.
.PP
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.
.PP
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" .
.PP
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.
.PP
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).
.PP
The bloom filter should be sized so that
.I nhash
\(ti
.I nblock
\(ti
0.7
\(<=
0.7 \(ti
.IR b ,
where
.I nblock
is the expected number of blocks stored on the server
and
.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
(see
.IR venti-fmt (8))
can be given either
.I nhash
or
.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.
.PP
The
.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.
.PP
The
.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.
.PP
The
.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.
.PP
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).
.PP
The
.I "bloom filter
requires memory equal to its size on disk,
as discussed above.
.PP
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 venti ,
80).
.PP
The venti web server provides the following
URLs for accessing status information:
.TP
.B /index
A summary of the usage of the arenas and index sections.
.TP
.B /xindex
An XML version of
.BR /index .
.TP
.B /storage
Brief storage totals.
.TP
.BI /set/ variable
The current integer value of
.IR variable .
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.
.TP
.BI /set/ variable / value
Set
.I variable
to
.IR value .
.TP
.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;
see
.B httpd.c
in the venti sources.
.TP
.B /log
A list of all debugging logs present in the server's memory.
.TP
.BI /log/ name
The contents of the debugging log with the given
.IR name .
.TP
.B /flushicache
Force venti to begin flushing the index cache to disk.
The request response will not be sent until the flush
has completed.
.TP
.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.
.PD
.PP
Requests for other files are served by consulting a
directory named in the configuration file
(see
.B webroot
below).
.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
with
.I fmtarenas
or
.I fmtisect
(see
.IR venti-fmt (8)).
In particular, only the configuration needs to be
changed if a component is moved to a different file.
.PP
The configuration file consists of lines in the form described below.
Lines starting with
.B #
are comments.
.TP
.BI index " name
Names the index for the system.
.TP
.BI arenas " file
.I File
is an arena partition, formatted using
.IR fmtarenas .
.TP
.BI isect " file
.I File
is an index section, formatted using
.IR fmtisect .
.PP
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;
run
.I fmtindex
with the
.B -a
flag to update the index.
.PP
The configuration file also holds configuration parameters
for the venti server itself.
These are:
.TF httpaddr netaddr
.TP
.BI mem " size
lump cache size
.TP
.BI bcmem " size
block cache size
.TP
.BI icmem " size
index cache size
.TP
.BI addr " netaddr
network address to announce venti service
(default
.BR tcp!*!venti )
.TP
.BI httpaddr " netaddr
network address to announce HTTP service
(default
.BR tcp!*!http )
.TP
.B queuewrites
queue writes in memory
(default is not to queue)
.TP
.BI webroot " dir
directory tree containing files for HTTP server
to consult for unrecognized URLs
.PD
.PP
The units for the various cache sizes above can be specified by appending a
.LR k ,
.LR m ,
or
.LR g
(case-insensitive)
to indicate kilobytes, megabytes, or gigabytes respectively.
.SS Command Line
Options to
.I venti
are:
.TP
.BI -c " config
The server configuration file
(default
.BR venti.conf )
.TP
.BI -o " line
Set a server parameter, using the same syntax
as in the configuration file.
The
.B -o
options override the configuration file.
.TP
.B -d
Produce various debugging information on standard error.
Implies
.BR -s .
.TP
.B -L
Enable logging. By default all logging is disabled.
Logging slows server operation considerably.
.TP
.B -s
Do not run in the background.
Normally,
the foreground process will exit once the Venti server
is initialized and ready for connections.
.PD
.SH EXAMPLE
A simple configuration:
.IP
.EX
% cat venti.conf
index main
isect /tmp/disks/isect0
isect /tmp/disks/isect1
arenas /tmp/disks/arenas
mem 10M
bcmem 20M
icmem 30M
%
.EE
.PP
Format the index sections, the arena partition, and
finally the main index:
.IP
.EX
% venti/fmtisect isect0. /tmp/disks/isect0 &
% venti/fmtisect isect1. /tmp/disks/isect1 &
% venti/fmtarenas arenas0. /tmp/disks/arenas &
% wait
% venti/fmtindex venti.conf
%
.EE
.PP
Start the server and check the storage statistics:
.IP
.EX
% venti/venti
% hget http://$sysname/storage
.EE
.SH "SEE ALSO"
.IR venti (1),
.IR venti (3),
.IR venti (7),
.IR venti-backup (8)
.IR venti-fmt (8)
.br
Sean Quinlan and Sean Dorward,
``Venti: a new approach to archival storage'',
.I "Usenix Conference on File and Storage Technologies" ,
2002.
.SH BUGS
Setting up a venti server is too complicated.
.PP
Venti should not require the user to decide how to
partition its memory usage.