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Date: Tue, 5 Apr 2011 16:39:05 +0000 (UTC)
From: "Filipe Manana (JIRA)" <jira@apache.org>
To: dev@couchdb.apache.org
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Subject: [jira] [Created] (COUCHDB-1120) Snappy compression (databases,
  view indexes) + keeping doc bodies as ejson binaries
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Snappy compression (databases,  view indexes) + keeping doc bodies as ejson binaries
------------------------------------------------------------------------------------

                 Key: COUCHDB-1120
                 URL: https://issues.apache.org/jira/browse/COUCHDB-1120
             Project: CouchDB
          Issue Type: Improvement
          Components: Database Core
         Environment: trunk
            Reporter: Filipe Manana
            Assignee: Filipe Manana


The branch at:

https://github.com/fdmanana/couchdb/compare/snappy

Is an experiment which adds snappy compression to database files and view index files. Snappy is a very fast compressor/decompressor developed by and used by Google [1] - even for small data chunks like 100Kb it can be 2 orders of magnitude faster then zlib or Erlang's term_to_binary compression level 1. Somewhere at [1] there are benchmark results published by Google that compare against zlib's deflate, Erlang's term_to_binary compression, lzo, etc.

Even small objects like database headers or btree nodes, still get smaller after compressing them with snappy, see the shell session at [2].
Besides the compression, this branch also keeps the document bodies (#doc.body fields) as binaries (snappy compressed ejson binaries) and only converts them back to ejson when absolutely needed (done by couch_doc:to_json_obj/2 for e.g.) - this is similar to COUCHDB-1092 - but the bodies are EJSON compressed binaries and doesn't suffer from the same issue Paul identified before (which could be fixed without many changes) - on reads we decompress and still do the binary_to_term/1 + ?JSON_ENCODE calls as before.

It also prepares the document summaries before sending the documents to the updater, so that we avoid copying EJSON terms and move this task outside of the updater to add more parallelism to concurrent updates.

I made some tests, comparing trunk before and after the JSON parser NIF was added, against this snappy branch.
I created databases with 1 000 000 documents of 4Kb each. The document template is this one:  http://friendpaste.com/qdfyId8w1C5vkxROc5Thf

The databases have this design document:

{
    "_id": "_design/test",
    "language": "javascript",
    "views": {
        "simple": {
            "map": "function(doc) { emit(doc.data5.float1, [doc.strings[2], doc.strings[10]]); }"
        }
    }
}

== Results with trunk ==

database file size after compaction:  7.5 Gb
view index file size after compaction:  257 Mb

** Before JSON nif:
$ time curl 'http://localhost:5985/trunk_db_1m/_design/test/_view/simple?limit=1'
{"total_rows":1000000,"offset":0,"rows":[
{"id":"00000632-d25d-49c6-9b4e-e038b78ff97d","key":76.572,"value":["jURcBZ0vrJcmf2roZUMzZJQoTsKZDIdj7KhO7itskKvM80jBU9","fKYYthv8iFvaYoFoYZyB"]}
]}

real	58m28.599s
user	0m0.036s
sys	0m0.056s

** After JSON nif:
fdmanana 12:45:55 /opt/couchdb > time curl 'http://localhost:5985/trunk_db_1m/_design/test/_view/simple?limit=1'
{"total_rows":1000000,"offset":0,"rows":[
{"id":"00000632-d25d-49c6-9b4e-e038b78ff97d","key":76.572,"value":["jURcBZ0vrJcmf2roZUMzZJQoTsKZDIdj7KhO7itskKvM80jBU9","fKYYthv8iFvaYoFoYZyB"]}
]}

real	51m14.738s
user	0m0.040s
sys	0m0.044s


== Results with the snappy branch ==

database file size after compaction:  3.2 Gb   (vs 7.5 Gb on trunk)
view index file size after compaction:  100 Mb  (vs 257 Mb on trunk)

** Before JSON nif:
$ time curl 'http://localhost:5984/snappy_db_1m/_design/test/_view/simple?limit=1'
{"total_rows":1000000,"offset":0,"rows":[
{"id":"00000632-d25d-49c6-9b4e-e038b78ff97d","key":76.572,"value":["jURcBZ0vrJcmf2roZUMzZJQoTsKZDIdj7KhO7itskKvM80jBU9","fKYYthv8iFvaYoFoYZyB"]}
]}

real	32m29.854s
user	0m0.008s
sys	0m0.052s

** After JSON nif:
fdmanana 15:40:39 /opt/couchdb > time curl 'http://localhost:5984/snappy_db_1m/_design/test/_view/simple?limit=1'
{"total_rows":1000000,"offset":0,"rows":[
{"id":"00000632-d25d-49c6-9b4e-e038b78ff97d","key":76.572,"value":["jURcBZ0vrJcmf2roZUMzZJQoTsKZDIdj7KhO7itskKvM80jBU9","fKYYthv8iFvaYoFoYZyB"]}
]}

real	18m39.240s
user	0m0.012s
sys	0m0.020s


A writes-only relaximation test also shows a significant improvement in the writes response times / throughput:

http://graphs.mikeal.couchone.com/#/graph/698bf36b6c64dbd19aa2bef63405480d

These results are also in a file of this branch [3].

Seems clear this, together with Paul's JSON NIF parser, has a very good impact in the view indexer, besides the big disk space savings and better write throughput.

Some potential issues:

* Snappy is C++, and so is the NIF [4] - however a C++ compiler is common and part of most development environments (gcc, xcode, etc)

* Not sure if snappy builds on Windows - it might build, it doesn't seem to depend on fancy libraries, just stdc++ and the STL

* Requires OTP R13B04 or higher. If built/running on R13B03 or below, it simple doesn't do any compression at all, just like current releases. However, 2 servers running this branch, one with R14 and other R13B01 for e.g., means that the second server will not be able to read database files created by the server with R14 - it will get an exception with the atom 'snappy_nif_not_loaded' - this is easy to catch and use for printing a nice and explicit error message to the user telling it needs to use a more recent otp release.

The upgrade of databases and view indexes from previous releases is done on compaction - I made just a few tests with database files by hand, this surely needs to be better tested.

Finally the branch is still in development phase, but maybe not far from completion, consider this ticket just as a way to share some results and get some feedback.

[1] - http://code.google.com/p/snappy/
[2] - http://friendpaste.com/45AOdi9MkFrS4BPsov7Lg8
[3] - https://github.com/fdmanana/couchdb/blob/b8f806e41727ba18ed6143cee31a3242e024ab2c/snappy-couch-tests.txt
[4] - https://github.com/fdmanana/snappy-erlang-nif/


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