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From "Duo Zhang (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-18055) HBASE-17917 closes the scanners while a scan is in progess for switching over to stream reads
Date Tue, 16 May 2017 07:08:04 GMT

    [ https://issues.apache.org/jira/browse/HBASE-18055?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16011859#comment-16011859

Duo Zhang commented on HBASE-18055:

U mean open new scanner with pread happens even before close of existing. Ya then how can
ref count go to 0?

There is a {{prevBlocks}} field in {{HFileScannerImpl}} which is used to track the HFileBlocks
we have scanned but not sent back to client yet. Create new scanner before closing old scanner
can only make sure that the ref count for the current HFileBlock is still being referenced,
but we will lost the information of the HFileBlocks stored in {{prevBlocks}}.

Let me think how to deal with this.

> HBASE-17917 closes the scanners while a scan is in progess for switching over to stream
> ---------------------------------------------------------------------------------------------
>                 Key: HBASE-18055
>                 URL: https://issues.apache.org/jira/browse/HBASE-18055
>             Project: HBase
>          Issue Type: Bug
>          Components: regionserver, Scanners
>    Affects Versions: 2.0.0
>            Reporter: ramkrishna.s.vasudevan
>            Assignee: ramkrishna.s.vasudevan
>             Fix For: 2.0.0
> In HBASE-17917 tries to switch from pread to stream read when a specific size of bytes
are read. So in order to switch over, it closes the existing scanners and creates a new scanners
with pread=false.
> When we close the exisitng scanners - if the blocks are served from offheap cache we
will decrement the ref count on those blocks and if it becomes zero we make the block ready
for eviction. Then there is a chance that the result could be corrupted if new blocks occupy
the cache. So the expectation was that till the RPC call completes the response we will hold
on to the blocks that are referred by the scan. (except the last one). So trying to switch
over to stream read will break this expectation and hence TestBlockEvictionfromclient fails.

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