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From "Adrien Grand (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (LUCENE-4509) Make CompressingStoredFieldsFormat the new default StoredFieldsFormat impl
Date Fri, 26 Oct 2012 21:29:12 GMT

    [ https://issues.apache.org/jira/browse/LUCENE-4509?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13485211#comment-13485211
] 

Adrien Grand commented on LUCENE-4509:
--------------------------------------

bq. Well you say you use a separate packed ints structure for the offsets right? so these
would all be zero?

These are absolute offsets in the fields data file. For example, when looking up a document,
it first performs a binary search in the first array (the one that contains the first document
IDs of every chunk). The resulting index is used to find the start offset of the chunk of
compressed documents thanks to the second array. When you read data starting at this offset
in the fields data file, there is first a packed ints array that stores the uncompressed length
of every document in the chunk, and then the compressed data. I'll add file formats docs soon...
                
> Make CompressingStoredFieldsFormat the new default StoredFieldsFormat impl
> --------------------------------------------------------------------------
>
>                 Key: LUCENE-4509
>                 URL: https://issues.apache.org/jira/browse/LUCENE-4509
>             Project: Lucene - Core
>          Issue Type: Wish
>          Components: core/store
>            Reporter: Adrien Grand
>            Priority: Minor
>
> What would you think of making CompressingStoredFieldsFormat the new default StoredFieldsFormat?
> Stored fields compression has many benefitsĀ :
>  - it makes the I/O cache work for us,
>  - file-based index replication/backup becomes cheaper.
> Things to know:
>  - even with incompressible data, there is less than 0.5% overhead with LZ4,
>  - LZ4 compression requires ~ 16kB of memory and LZ4 HC compression requires ~ 256kB,
>  - LZ4 uncompression has almost no memory overhead,
>  - on my low-end laptop, the LZ4 impl in Lucene uncompresses at ~ 300mB/s.
> I think we could use the same default parameters as in CompressingCodec :
>  - LZ4 compression,
>  - in-memory stored fields index that is very memory-efficient (less than 12 bytes per
block of compressed docs) and uses binary search to locate documents in the fields data file,
>  - 16 kB blocks (small enough so that there is no major slow down when the whole index
would fit into the I/O cache anyway, and large enough to provide interesting compression ratiosĀ ;
for example Robert got a 0.35 compression ratio with the geonames.org database).
> Any concerns?

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