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From "Andrew Purtell (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-14283) Reverse scan doesn’t work with HFile inline index/bloom blocks
Date Sat, 26 Sep 2015 17:29:04 GMT

    [ https://issues.apache.org/jira/browse/HBASE-14283?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14909362#comment-14909362
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Andrew Purtell commented on HBASE-14283:
----------------------------------------

When we were designing tags we accepted some limitations of HFile that later were problematic,
specifically the issue of file and Cell serialized form bloat produced by adding the tag length,
even if universally 0 for the file. We introduced a whole file optimization for this issue
but clearly we'd have more opportunities to employ it if we could vary encoding strategy on
a block-by-block basis. I thought about introducing an extensible pbufed block header. It
didn't make sense for the tag serialization issue - we would trade one type of bloat for another,
those additional header bytes will end up in the block cache - but if there are multiple use
cases for it lined up, a new extensible pbufed 'block header' could be worthwhile. Would make
future block level changes less likely to be incompatible changes too. 

Does the introduction of something like that require a major or minor version bump? I think
so. I'd like to see us be more like semver with HFile versioning, and if we're on the same
page about that, this is a major version bump because earlier versioned readers won't be able
to handle the change. That is assuming 

> 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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