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From "Andrzej Bialecki (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (LUCENE-4050) Make segments_NN file codec-independent
Date Tue, 15 May 2012 23:15:06 GMT

    [ https://issues.apache.org/jira/browse/LUCENE-4050?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13276325#comment-13276325
] 

Andrzej Bialecki  commented on LUCENE-4050:
-------------------------------------------

bq. The only small thing we lose is if a disk full is going to strike... 
I thought about this too - if it's really a big concern we could use the following trick:
> 99% filesystems keep data in blocks that are multiples of 512 bytes. We could add filler
bytes at the end of the file so that it comes out to a round multiple of 512 B, and only then
append the marker and the checksum. This way we will know that writing a marker required allocation
of a new block, and if it succeeded then writing a checksum should also succeed.
                
> Make segments_NN file codec-independent
> ---------------------------------------
>
>                 Key: LUCENE-4050
>                 URL: https://issues.apache.org/jira/browse/LUCENE-4050
>             Project: Lucene - Java
>          Issue Type: Bug
>          Components: core/codecs
>            Reporter: Andrzej Bialecki 
>            Assignee: Robert Muir
>             Fix For: 4.0
>
>
> I propose to change the format of SegmentInfos file (segments_NN) to use plain text instead
of the current binary format.
> SegmentInfos file represents a commit point, and it also declares what codecs were used
for writing each of the segments that the commit point consists of. However, this is a chicken
and egg situation - in theory the format of this file is customizable via Codec.getSegmentInfosFormat,
but in practice we have to first discover what is the codec implementation that wrote this
file - so the SegmentCoreReaders assumes a certain fixed binary layout of a preamble of this
file that contains the codec name... and then the file is read again, only this time using
the right Codec.
> This is ugly. Instead I propose to use a simple plain text format, either line oriented
properties or JSON, in such a way that newer versions could easily extend it, and which wouldn't
require any special Codec to read and parse. Consequently we could remove SegmentInfosFormat
altogether, and instead add SegmentInfoFormat (notice the singular) to Codec to read single
per-segment SegmentInfo-s in a codec-specific way. E.g. for Lucene40 codec we could either
add another file or we could extend the .fnm file (FieldInfos) to contain also this information.

> Then the plain text SegmentInfos would contain just the following information:
> * list of global files for this commit point (if any)
> * list of segments for this commit point, and their corresponding codec class names
> * user data map

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