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From "Anoop Sam John (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-19468) FNFE during scans and flushes
Date Mon, 11 Dec 2017 10:47:02 GMT

    [ https://issues.apache.org/jira/browse/HBASE-19468?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16285747#comment-16285747
] 

Anoop Sam John commented on HBASE-19468:
----------------------------------------

May be in below scenario issue will come?
We are in btw a read as said in issue desc and got some data at client side and processing
Mean while in server one flush happened. This file is added to the flushedStoreFiles.  
Again one more flush and this is also added
One more flush and this is also added
And this makes 3 small files and assume a compaction started and completed. 3 new files are
compacted away
Remember the scanner did not call any seek or next yet and so no processing of the 3 new flushed
files happened. Means no scanners were obtained on them and so ref count on 3 files are still
0 (?)
The CompactedHFilesDischarger may be able to remove these files then as they are not yet referenced!
Now if a next() happens, still these 3 files are there in 'flushedStoreFiles' and if tried
to make scanners, we will get FNFE.
Just on a quick look at related code am asking. Pls correct if wrong.

> FNFE during scans and flushes
> -----------------------------
>
>                 Key: HBASE-19468
>                 URL: https://issues.apache.org/jira/browse/HBASE-19468
>             Project: HBase
>          Issue Type: Sub-task
>          Components: regionserver, Scanners
>    Affects Versions: 1.3.1
>            Reporter: Thiruvel Thirumoolan
>            Priority: Critical
>             Fix For: 2.0.0, 1.4.1, 1.5.0, 1.3.3
>
>
> We see FNFE exceptions on our 1.3 clusters when scans and flushes happen at the same
time. This causes regionserver to throw a UnknownScannerException and client retries.
> This happens during the following sequence:
> 1. Scanner open, client fetched some rows from regionserver and working on it
> 2. Flush happens and storeScanner is updated with flushed files (StoreScanner.updateReaders())
> 3. Compaction discharger runs and cleans up the newly flushed file as we don't have new
scanners on it yet.
> 4. Client issues scan.next and during StoreScanner.resetScannerStack(), we get a FNFE.
RegionServer throws a UnknownScannerThe client retries in 1.3. With branch-1.4, the scan fails
with a DoNotRetryIOException.
> [~ram_krish], My proposal is to increment the reader count during updateReaders() and
decrement it during resetScannerStack(), so discharger doesn't clean it up. Scan lease expiries
also have to be taken care of. Am I missing anything? Is there a better approach?



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