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From "Filipe Manana (JIRA)" <>
Subject [jira] Commented: (COUCHDB-1092) Storing documents bodies as raw JSON binaries instead of serialized JSON terms
Date Wed, 16 Mar 2011 13:51:30 GMT


Filipe Manana commented on COUCHDB-1092:

One more test, this time with 100 000 documents having no floats at all. Template is the following:

(removed all dots to get integers instead)

With current trunk after compaction I get:

$ du -m trunk_db.couch 
936	trunk_db.couch

$ time curl http://localhost:5985/trunk_db/_design/test/_view/simple?limit=1

real	4m18.466s
user	0m0.000s
sys	0m0.008s

Now using my branch without compression:

$ du -m branch_nozip_db.couch 
1184	branch_nozip_db.couch

$ time curl http://localhost:5984/branch_nozip_db/_design/test/_view/simple?limit=1

real	3m38.747s
user	0m0.000s
sys	0m0.012s

(The database size is oddly bigger, can't figure out why, possibly did something wrong)

With document body compression:

$ du -m branch_db.couch 
297	branch_db.couch

$ time curl http://localhost:5984/branch_db/_design/test/_view/simple?limit=1

real	3m1.282s
user	0m0.012s
sys	0m0.000s

The database size is much smaller and building the view index from scratch takes about the
same time.

Relaximation tests with and without document body compression:

With compression:
Without compression:

In both cases write performance seems to be significantly better.

I repeated the test with the 11Kb documents that have floats, with and without the compression:

With compression (100 000 11kb docs with floats):

$ du -m branch_floats_zip_db.couch
297	branch_floats_zip_db.couch

$ time curl http://localhost:5984/branch_floats_zip_db/_design/test/_view/simple?limit=1

real	2m46.847s
user	0m0.012s
sys	0m0.004s

No compression (100 000 11kb docs with floats):

$ du -m branch_floats_nozip_db.couch 
1184	branch_floats_nozip_db.couch

$ time curl http://localhost:5984/branch_floats_nozip_db/_design/test/_view/simple?limit=1

real	3m12.053s
user	0m0.004s
sys	0m0.012s

This makes me think that compressing the json bodies doesn't give worse results compared to
first approach. And it's still way better then trunk, the database got reduced from 2202Mb
to 297Mb.

I've also moved now the compression to outside of the updater process. Relaximation test gives
about the same results as before - still a significant boost in write performance.

> Storing documents bodies as raw JSON binaries instead of serialized JSON terms
> ------------------------------------------------------------------------------
>                 Key: COUCHDB-1092
>                 URL:
>             Project: CouchDB
>          Issue Type: Improvement
>          Components: Database Core
>            Reporter: Filipe Manana
>            Assignee: Filipe Manana
> Currently we store documents as Erlang serialized (via the term_to_binary/1 BIF) EJSON.
> The proposed patch changes the database file format so that instead of storing serialized
> EJSON document bodies, it stores raw JSON binaries.
> The github branch is at:
> Advantages:
> * what we write to disk is much smaller - a raw JSON binary can easily get up to 50%
>   (at least according to the tests I did)
> * when serving documents to a client we no longer need to JSON encode the document body
>   read from the disk - this applies to individual document requests, view queries with
>   ?include_docs=true, pull and push replications, and possibly other use cases.
>   We just grab its body and prepend the _id, _rev and all the necessary metadata fields
>   (this is via simple Erlang binary operations)
> * we avoid the EJSON term copying between request handlers and the db updater processes,
>   between the work queues and the view updater process, between replicator processes,
> * before sending a document to the JavaScript view server, we no longer need to convert
>   from EJSON to JSON
> The changes done to the document write workflow are minimalist - after JSON decoding
> document's JSON into EJSON and removing the metadata top level fields (_id, _rev, etc),
> JSON encodes the resulting EJSON body into a binary - this consumes CPU of course but
> brings 2 advantages:
> 1) we avoid the EJSON copy between the request process and the database updater process
>    for any realistic document size (4kb or more) this can be very expensive, specially
>    when there are many nested structures (lists inside objects inside lists, etc)
> 2) before writing anything to the file, we do a term_to_binary([Len, Md5, TheThingToWrite])
>    and then write the result to the file. A term_to_binary call with a binary as the
>    is very fast compared to a term_to_binary call with EJSON as input (or some other
>    structure)
> I think both compensate the JSON encoding after the separation of meta data fields and
non-meta data fields.
> The following relaximation graph, for documents with sizes of 4Kb, shows a significant
> performance increase both for writes and reads - especially reads.   
> I've also made a few tests to see how much the improvement is when querying a view, for
> first time, without ?stale=ok. The size difference of the databases (after compaction)
> also very significant - this change can reduce the size at least 50% in common cases.
> The test databases were created in an instance built from that experimental branch.
> Then they were replicated into a CouchDB instance built from the current trunk.
> At the end both databases were compacted (to fairly compare their final sizes).
> The databases contain the following view:
> {
>     "_id": "_design/test",
>     "language": "javascript",
>     "views": {
>         "simple": {
>             "map": "function(doc) { emit(doc.float1, doc.strings[1]); }"
>         }
>     }
> }
> ## Database with 500 000 docs of 2.5Kb each
> Document template is at:
> Sizes (branch vs trunk):
> $ du -m couchdb/tmp/lib/disk_json_test.couch 
> 1996	couchdb/tmp/lib/disk_json_test.couch
> $ du -m couchdb-trunk/tmp/lib/disk_ejson_test.couch 
> 2693	couchdb-trunk/tmp/lib/disk_ejson_test.couch
> Time, from a user's perpective, to build the view index from scratch:
> $ time curl http://localhost:5984/disk_json_test/_design/test/_view/simple?limit=1
> {"total_rows":500000,"offset":0,"rows":[
> {"id":"0000076a-c1ae-4999-b508-c03f4d0620c5","key":null,"value":"wfxuF3N8XEK6"}
> ]}
> real	6m6.740s
> user	0m0.016s
> sys	0m0.008s
> $ time curl http://localhost:5985/disk_ejson_test/_design/test/_view/simple?limit=1
> {"total_rows":500000,"offset":0,"rows":[
> {"id":"0000076a-c1ae-4999-b508-c03f4d0620c5","key":null,"value":"wfxuF3N8XEK6"}
> ]}
> real	15m41.439s
> user	0m0.012s
> sys	0m0.012s
> ## Database with 100 000 docs of 11Kb each
> Document template is at:
> Sizes (branch vs trunk):
> $ du -m couchdb/tmp/lib/disk_json_test_11kb.couch
> 1185	couchdb/tmp/lib/disk_json_test_11kb.couch
> $ du -m couchdb-trunk/tmp/lib/disk_ejson_test_11kb.couch
> 2202	couchdb-trunk/tmp/lib/disk_ejson_test_11kb.couch
> Time, from a user's perpective, to build the view index from scratch:
> $ time curl http://localhost:5984/disk_json_test_11kb/_design/test/_view/simple?limit=1
> {"total_rows":100000,"offset":0,"rows":[
> {"id":"00001511-831c-41ff-9753-02861bff73b3","key":null,"value":"2fQUbzRUax4A"}
> ]}
> real	4m19.306s
> user	0m0.008s
> sys	0m0.004s
> $ time curl http://localhost:5985/disk_ejson_test_11kb/_design/test/_view/simple?limit=1
> {"total_rows":100000,"offset":0,"rows":[
> {"id":"00001511-831c-41ff-9753-02861bff73b3","key":null,"value":"2fQUbzRUax4A"}
> ]}
> real	18m46.051s
> user	0m0.008s
> sys	0m0.016s
> All in all, I haven't seen yet any disadvantage with this approach. Also, the code changes
> don't bring additional complexity. I say the performance and disk space gains it gives
> very positive.
> This branch still needs to be polished in a few places. But I think it isn't far from
getting mature.
> Other experiments that can be done are to store view values as raw JSON binaries as well
(instead of EJSON)
> and optional compression of the stored JSON binaries (since it's pure text, the compression
ratio is very high).
> However, I would prefer to do these other 2 suggestions in separate branches/patches
- I haven't actually tested
> any of them yet, so maybe they not bring significant gains.
> Thoughts? :)

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