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From "Ben Lau (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-14283) Reverse scan doesn’t work with HFile inline index/bloom blocks
Date Wed, 09 Sep 2015 03:33:46 GMT

    [ https://issues.apache.org/jira/browse/HBASE-14283?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14736093#comment-14736093
] 

Ben Lau commented on HBASE-14283:
---------------------------------

Yep, we talked with Anoop and agree that the patch adds a lot of complexity for a fix that
doesn't fix the issue 100%.  The portion of the patch that is required to fix the bug for
bloom filters is especially long.  We thought to aim for a longer term fix later, but based
on our discussion with Anoop it sounds like a backwards compatible, complete fix that adds
the necessary metadata to HFile should not be too complicated/much work (eg does not involve
creating new HFileReader implementation or other infrastructure).  We will submit a new patch
later with a final fix.  We will keep the unit tests from the 1st patch since they are still
applicable.

> Reverse scan doesn’t work with HFile inline index/bloom blocks
> --------------------------------------------------------------
>
>                 Key: HBASE-14283
>                 URL: https://issues.apache.org/jira/browse/HBASE-14283
>             Project: HBase
>          Issue Type: Bug
>            Reporter: Ben Lau
>            Assignee: Ben Lau
>         Attachments: HBASE-14283-v2.patch, HBASE-14283.patch, hfile-seek-before.patch
>
>
> Reverse scans do not work if an HFile contains inline bloom blocks or leaf level index
blocks.  The reason is because the seekBefore() call calculates the previous data block’s
size by assuming data blocks are contiguous which is not the case in HFile V2 and beyond.
> Attached is a first cut patch (targeting bcef28eefaf192b0ad48c8011f98b8e944340da5 on
trunk) which includes:
> (1) a unit test which exposes the bug and demonstrates failures for both inline bloom
blocks and inline index blocks
> (2) a proposed fix for inline index blocks that does not require a new HFile version
change, but is only performant for 1 and 2-level indexes and not 3+.  3+ requires an HFile
format update for optimal performance.    
> This patch does not fix the bloom filter blocks bug.  But the fix should be similar to
the case of inline index blocks.  The reason I haven’t made the change yet is I want to
confirm that you guys would be fine with me revising the HFile.Reader interface.
> Specifically, these 2 functions (getGeneralBloomFilterMetadata and getDeleteBloomFilterMetadata)
need to return the BloomFilter.  Right now the HFileReader class doesn’t have a reference
to the bloom filters (and hence their indices) and only constructs the IO streams and hence
has no way to know where the bloom blocks are in the HFile.  It seems that the HFile.Reader
bloom method comments state that they “know nothing about how that metadata is structured”
but I do not know if that is a requirement of the abstraction (why?) or just an incidental
current property. 
> We would like to do 3 things with community approval:
> (1) Update the HFile.Reader interface and implementation to contain and return BloomFilters
directly rather than unstructured IO streams
> (2) Merge the fixes for index blocks and bloom blocks into open source
> (3) Create a new Jira ticket for open source HBase to add a ‘prevBlockSize’ field
in the block header in the next HFile version, so that seekBefore() calls can not only be
correct but performant in all cases.



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