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From "ramkrishna.s.vasudevan (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-15477) Do not save 'next block header' when we cache hfileblocks
Date Tue, 22 Mar 2016 06:59:25 GMT

    [ https://issues.apache.org/jira/browse/HBASE-15477?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15205914#comment-15205914
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ramkrishna.s.vasudevan commented on HBASE-15477:
------------------------------------------------

bq.We don't save the 'header' but we do save the length of the next block which is the important
bit of info to have (for then we can read the next block in one seek... the next block's header
comes in as part of this seek and will be sitting in the appropriate place in the buffer –
no need to do the back-filling header tricks).
Applying the patch I can see this happening now. So in bucket cache we still write the header
info of the next bytes to make use of that optimization. 

> Do not save 'next block header' when we cache hfileblocks
> ---------------------------------------------------------
>
>                 Key: HBASE-15477
>                 URL: https://issues.apache.org/jira/browse/HBASE-15477
>             Project: HBase
>          Issue Type: Sub-task
>          Components: BlockCache, Performance
>            Reporter: stack
>            Assignee: stack
>         Attachments: 15366v4.patch, 15477.patch, 15477v2.patch, 15477v3.patch, 15477v3.patch,
15477v4.patch
>
>
>     When we read from HDFS, we overread to pick up the next blocks header.
>     Doing this saves a seek as we move through the hfile; we save having to
>     do an explicit seek just to read the block header every time we need to
>     read the body.  We used to read in the next header as part of the
>     current blocks buffer. This buffer was then what got persisted to
>     blockcache; so we were over-persisting wrtiting out our block plus the
>     next blocks' header (overpersisting 33 bytes). Parse of HFileBlock
>     complicated by this extra tail. Fix.



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