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tventi.8 (12887B)
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1 .TH VENTI 8
2 .SH NAME
3 venti \- archival storage server
4 .SH SYNOPSIS
5 .in +0.25i
6 .ti -0.25i
7 .B venti/venti
8 [
9 .B -Ldrs
10 ]
11 [
12 .B -a
13 .I address
14 ]
15 [
16 .B -B
17 .I blockcachesize
18 ]
19 [
20 .B -c
21 .I config
22 ]
23 [
24 .B -C
25 .I lumpcachesize
26 ]
27 [
28 .B -h
29 .I httpaddress
30 ]
31 [
32 .B -I
33 .I indexcachesize
34 ]
35 [
36 .B -W
37 .I webroot
38 ]
39 .SH DESCRIPTION
40 .I Venti
41 is a SHA1-addressed archival storage server.
42 See
43 .MR venti (7)
44 for a full introduction to the system.
45 This page documents the structure and operation of the server.
46 .PP
47 A venti server requires multiple disks or disk partitions,
48 each of which must be properly formatted before the server
49 can be run.
50 .SS Disk
51 The venti server maintains three disk structures, typically
52 stored on raw disk partitions:
53 the append-only
54 .IR "data log" ,
55 which holds, in sequential order,
56 the contents of every block written to the server;
57 the
58 .IR index ,
59 which helps locate a block in the data log given its score;
60 and optionally the
61 .IR "bloom filter" ,
62 a concise summary of which scores are present in the index.
63 The data log is the primary storage.
64 To improve the robustness, it should be stored on
65 a device that provides RAID functionality.
66 The index and the bloom filter are optimizations
67 employed to access the data log efficiently and can be rebuilt
68 if lost or damaged.
69 .PP
70 The data log is logically split into sections called
71 .IR arenas ,
72 typically sized for easy offline backup
73 (e.g., 500MB).
74 A data log may comprise many disks, each storing
75 one or more arenas.
76 Such disks are called
77 .IR "arena partitions" .
78 Arena partitions are filled in the order given in the configuration.
79 .PP
80 The index is logically split into block-sized pieces called
81 .IR buckets ,
82 each of which is responsible for a particular range of scores.
83 An index may be split across many disks, each storing many buckets.
84 Such disks are called
85 .IR "index sections" .
86 .PP
87 The index must be sized so that no bucket is full.
88 When a bucket fills, the server must be shut down and
89 the index made larger.
90 Since scores appear random, each bucket will contain
91 approximately the same number of entries.
92 Index entries are 40 bytes long. Assuming that a typical block
93 being written to the server is 8192 bytes and compresses to 4096
94 bytes, the active index is expected to be about 1% of
95 the active data log.
96 Storing smaller blocks increases the relative index footprint;
97 storing larger blocks decreases it.
98 To allow variation in both block size and the random distribution
99 of scores to buckets, the suggested index size is 5% of
100 the active data log.
101 .PP
102 The (optional) bloom filter is a large bitmap that is stored on disk but
103 also kept completely in memory while the venti server runs.
104 It helps the venti server efficiently detect scores that are
105 .I not
106 already stored in the index.
107 The bloom filter starts out zeroed.
108 Each score recorded in the bloom filter is hashed to choose
109 .I nhash
110 bits to set in the bloom filter.
111 A score is definitely not stored in the index of any of its
112 .I nhash
113 bits are not set.
114 The bloom filter thus has two parameters:
115 .I nhash
116 (maximum 32)
117 and the total bitmap size
118 (maximum 512MB, 2\s-2\u32\d\s+2 bits).
119 .PP
120 The bloom filter should be sized so that
121 .I nhash
122 \(mu
123 .I nblock
124 \(<=
125 0.7 \(mu
126 .IR b ,
127 where
128 .I nblock
129 is the expected number of blocks stored on the server
130 and
131 .I b
132 is the bitmap size in bits.
133 The false positive rate of the bloom filter when sized
134 this way is approximately 2\s-2\u\-\fInblock\fR\d\s+2.
135 .I Nhash
136 less than 10 are not very useful;
137 .I nhash
138 greater than 24 are probably a waste of memory.
139 .I Fmtbloom
140 (see
141 .MR venti-fmt (8) )
142 can be given either
143 .I nhash
144 or
145 .IR nblock ;
146 if given
147 .IR nblock ,
148 it will derive an appropriate
149 .IR nhash .
150 .SS Memory
151 Venti can make effective use of large amounts of memory
152 for various caches.
153 .PP
154 The
155 .I "lump cache
156 holds recently-accessed venti data blocks, which the server refers to as
157 .IR lumps .
158 The lump cache should be at least 1MB but can profitably be much larger.
159 The lump cache can be thought of as the level-1 cache:
160 read requests handled by the lump cache can
161 be served instantly.
162 .PP
163 The
164 .I "block cache
165 holds recently-accessed
166 .I disk
167 blocks from the arena partitions.
168 The block cache needs to be able to simultaneously hold two blocks
169 from each arena plus four blocks for the currently-filling arena.
170 The block cache can be thought of as the level-2 cache:
171 read requests handled by the block cache are slower than those
172 handled by the lump cache, since the lump data must be extracted
173 from the raw disk blocks and possibly decompressed, but no
174 disk accesses are necessary.
175 .PP
176 The
177 .I "index cache
178 holds recently-accessed or prefetched
179 index entries.
180 The index cache needs to be able to hold index entries
181 for three or four arenas, at least, in order for prefetching
182 to work properly. Each index entry is 50 bytes.
183 Assuming 500MB arenas of
184 128,000 blocks that are 4096 bytes each after compression,
185 the minimum index cache size is about 6MB.
186 The index cache can be thought of as the level-3 cache:
187 read requests handled by the index cache must still go
188 to disk to fetch the arena blocks, but the costly random
189 access to the index is avoided.
190 .PP
191 The size of the index cache determines how long venti
192 can sustain its `burst' write throughput, during which time
193 the only disk accesses on the critical path
194 are sequential writes to the arena partitions.
195 For example, if you want to be able to sustain 10MB/s
196 for an hour, you need enough index cache to hold entries
197 for 36GB of blocks. Assuming 8192-byte blocks,
198 you need room for almost five million index entries.
199 Since index entries are 50 bytes each, you need 250MB
200 of index cache.
201 If the background index update process can make a single
202 pass through the index in an hour, which is possible,
203 then you can sustain the 10MB/s indefinitely (at least until
204 the arenas are all filled).
205 .PP
206 The
207 .I "bloom filter
208 requires memory equal to its size on disk,
209 as discussed above.
210 .PP
211 A reasonable starting allocation is to
212 divide memory equally (in thirds) between
213 the bloom filter, the index cache, and the lump and block caches;
214 the third of memory allocated to the lump and block caches
215 should be split unevenly, with more (say, two thirds)
216 going to the block cache.
217 .SS Network
218 The venti server announces two network services, one
219 (conventionally TCP port
220 .BR venti ,
221 17034) serving
222 the venti protocol as described in
223 .MR venti (7) ,
224 and one serving HTTP
225 (conventionally TCP port
226 .BR http ,
227 80).
228 .PP
229 The venti web server provides the following
230 URLs for accessing status information:
231 .TF "\fL/storage"
232 .PD
233 .TP
234 .B /index
235 A summary of the usage of the arenas and index sections.
236 .TP
237 .B /xindex
238 An XML version of
239 .BR /index .
240 .TP
241 .B /storage
242 Brief storage totals.
243 .TP
244 .BI /set
245 Disable the values of all variables.
246 Variables are:
247 .BR compress ,
248 whether or not to compress blocks
249 (for debugging);
250 .BR logging ,
251 whether to write entries to the debugging logs;
252 .BR stats ,
253 whether to collect run-time statistics;
254 .BR icachesleeptime ,
255 the time in milliseconds between successive updates
256 of megabytes of the index cache;
257 .BR arenasumsleeptime ,
258 the time in milliseconds between reads while
259 checksumming an arena in the background.
260 The two sleep times should be (but are not) managed by venti;
261 they exist to provide more experience with their effects.
262 The other variables exist only for debugging and
263 performance measurement.
264 .TP
265 .BI /set?name= variable
266 Show the current setting of
267 .IR variable .
268 .TP
269 .BI /set?name= variable &value= value
270 Set
271 .I variable
272 to
273 .IR value .
274 .TP
275 .BI /graph?arg= name [&arg2= name] &graph= type &param= value \fR...
276 A PNG image graphing the
277 .IT name
278 run-time statistic over time.
279 The details of names and parameters are mostly undocumented;
280 see the
281 .BR graphname
282 array in
283 .B httpd.c
284 in the venti code for a list of possible statistics. The
285 .IR type
286 of graph defaults to raw, see the
287 .BR xgraph
288 function for a list of types. Possible
289 .IR param
290 include the timeframe
291 .BR (t0
292 and
293 .BR t1)
294 , the y limits
295 .BR (min
296 and
297 .BR max)
298 etc.
299 .TP
300 .B /log
301 A list of all debugging logs present in the server's memory.
302 .TP
303 .BI /log/ name
304 The contents of the debugging log with the given
305 .IR name .
306 .TP
307 .B /flushicache
308 Force venti to begin flushing the index cache to disk.
309 The request response will not be sent until the flush
310 has completed.
311 .TP
312 .B /flushdcache
313 Force venti to begin flushing the arena block cache to disk.
314 The request response will not be sent until the flush
315 has completed.
316 .PD
317 .PP
318 Requests for other files are served by consulting a
319 directory named in the configuration file
320 (see
321 .B webroot
322 below).
323 .SS Configuration File
324 A venti configuration file
325 enumerates the various index sections and
326 arenas that constitute a venti system.
327 The components are indicated by the name of the file, typically
328 a disk partition, in which they reside. The configuration
329 file is the only location that file names are used. Internally,
330 venti uses the names assigned when the components were formatted
331 with
332 .I fmtarenas
333 or
334 .I fmtisect
335 (see
336 .MR venti-fmt (8) ).
337 In particular, only the configuration needs to be
338 changed if a component is moved to a different file.
339 .PP
340 The configuration file consists of lines in the form described below.
341 Lines starting with
342 .B #
343 are comments.
344 .TF "\fLindex\fI name "
345 .PD
346 .TP
347 .BI index " name
348 Names the index for the system.
349 .TP
350 .BI arenas " file
351 .I File
352 is an arena partition, formatted using
353 .IR fmtarenas .
354 .TP
355 .BI isect " file
356 .I File
357 is an index section, formatted using
358 .IR fmtisect .
359 .TP
360 .BI bloom " file
361 .I File
362 is a bloom filter, formatted using
363 .IR fmtbloom .
364 .PD
365 .PP
366 After formatting a venti system using
367 .IR fmtindex ,
368 the order of arenas and index sections should not be changed.
369 Additional arenas can be appended to the configuration;
370 run
371 .I fmtindex
372 with the
373 .B -a
374 flag to update the index.
375 .PP
376 The configuration file also holds configuration parameters
377 for the venti server itself.
378 These are:
379 .TF "\fLhttpaddr\fI netaddr "
380 .TP
381 .BI mem " size
382 lump cache size
383 .TP
384 .BI bcmem " size
385 block cache size
386 .TP
387 .BI icmem " size
388 index cache size
389 .TP
390 .BI addr " netaddr
391 network address to announce venti service
392 (default
393 .BR tcp!*!venti )
394 .TP
395 .BI httpaddr " netaddr
396 network address to announce HTTP service
397 (default is not to start the service)
398 .TP
399 .B queuewrites
400 queue writes in memory
401 (default is not to queue)
402 .TP
403 .BI webroot " dir
404 directory tree containing files for
405 .IR venti 's
406 internal HTTP server to consult for unrecognized URLs
407 .PD
408 .PP
409 The units for the various cache sizes above can be specified by appendin…
410 .LR k ,
411 .LR m ,
412 or
413 .LR g
414 (case-insensitive)
415 to indicate kilobytes, megabytes, or gigabytes respectively.
416 .PP
417 The
418 .I file
419 name in the configuration lines above can be of the form
420 .IB file : lo - hi
421 to specify a range of the file.
422 .I Lo
423 and
424 .I hi
425 are specified in bytes but can have the usual
426 .BI k ,
427 .BI m ,
428 or
429 .B g
430 suffixes.
431 Either
432 .I lo
433 or
434 .I hi
435 may be omitted.
436 This notation eliminates the need to
437 partition raw disks on non-Plan 9 systems.
438 .SS Command Line
439 Many of the options to Venti duplicate parameters that
440 can be specified in the configuration file.
441 The command line options override those found in a
442 configuration file.
443 Additional options are:
444 .TF "\fL-c\fI config"
445 .PD
446 .TP
447 .BI -c " config
448 The server configuration file
449 (default
450 .BR venti.conf )
451 .TP
452 .B -d
453 Produce various debugging information on standard error.
454 Implies
455 .BR -s .
456 .TP
457 .B -L
458 Enable logging. By default all logging is disabled.
459 Logging slows server operation considerably.
460 .TP
461 .B -r
462 Allow only read access to the venti data.
463 .TP
464 .B -s
465 Do not run in the background.
466 Normally,
467 the foreground process will exit once the Venti server
468 is initialized and ready for connections.
469 .PD
470 .SH EXAMPLE
471 A simple configuration:
472 .IP
473 .EX
474 % cat venti.conf
475 index main
476 isect /tmp/disks/isect0
477 isect /tmp/disks/isect1
478 arenas /tmp/disks/arenas
479 bloom /tmp/disks/bloom
480 mem 10M
481 bcmem 20M
482 icmem 30M
483 %
484 .EE
485 .PP
486 Format the index sections, the arena partition,
487 the bloom filter, and
488 finally the main index:
489 .IP
490 .EX
491 % venti/fmtisect isect0. /tmp/disks/isect0
492 % venti/fmtisect isect1. /tmp/disks/isect1
493 % venti/fmtarenas arenas0. /tmp/disks/arenas &
494 % venti/fmtbloom /tmp/disks/bloom &
495 % wait
496 % venti/fmtindex venti.conf
497 %
498 .EE
499 .PP
500 Start the server and check the storage statistics:
501 .IP
502 .EX
503 % venti/venti
504 % hget http://$sysname/storage
505 .EE
506 .SH SOURCE
507 .B \*9/src/cmd/venti/srv
508 .SH "SEE ALSO"
509 .MR venti (1) ,
510 .MR venti (3) ,
511 .MR venti (7) ,
512 .MR venti-backup (8)
513 .MR venti-fmt (8)
514 .br
515 Sean Quinlan and Sean Dorward,
516 ``Venti: a new approach to archival storage'',
517 .I "Usenix Conference on File and Storage Technologies" ,
518 2002.
519 .SH BUGS
520 Setting up a venti server is too complicated.
521 .PP
522 Venti should not require the user to decide how to
523 partition its memory usage.
524 .PP
525 Users of shells other than
526 .MR rc (1)
527 will not be able to use the program names shown.
528 One solution is to define
529 .B "V=$PLAN9/bin/venti"
530 and then substitute
531 .B $V/
532 for
533 .B venti/
534 in the paths above.
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