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 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.

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.

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 information:
/index     A summary of the usage of the arenas and index sections.
/xindex    An XML version of /index.
/storage   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 variables 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.
/log/nameThe 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 completed.

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 tcp!*!http)
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.
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 parameters, including those on the command line and in the
configuration 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 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

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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