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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, 07 Oct 2015 04:07:27 GMT

    [ https://issues.apache.org/jira/browse/HBASE-14283?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14946197#comment-14946197
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Ben Lau commented on HBASE-14283:
---------------------------------

Hey guys, sorry, I should be able to get back to this soon.  Finishing up an unrelated project
right now.  I didn't know that minor versions in HFiles were also non-backwards compatible.
 That's one less reason then to make this a major version bump.  If anyone has a strong preference
for this fix to go into a V3.X I can change the patch to use minor version (eg for header
size calculation) when I have time to do it.  If not I'll leave it as V4 since it's a little
simpler in the code as a major version bump.  My original intention btw if it wasn't clear
was that this wouldn't be the only change in a V4, just the first change that would go into
a V4, whose format/contents is not yet meant to be final even when this patch is committed,
i.e. V4 would be essentially a WIP with more changes suggested and implemented in other tickets
and eventually released in HBase 2.0.

[~anoop.hbase] I'm down for committing a short-term read-the-header-always fix for now and
then discussing the longer term solution second.  Which branches do you want the patch for?

> 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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