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From "stack (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-8741) Mutations on Regions in recovery mode might have same sequenceIDs
Date Sat, 22 Jun 2013 04:49:20 GMT

    [ https://issues.apache.org/jira/browse/HBASE-8741?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13691050#comment-13691050
] 

stack commented on HBASE-8741:
------------------------------

[~sershe] We would have to have a running Map per WAL.  This map would be keyed by region
name.  Its value would be the max edit number written to the WAL.  We would have to update
this number at write time. It can be in-memory only since on crash, the WALs will be replayed
and removed.  We already keep minimum edit id by region.  On a period we would look at a WAL
and all the regions it had mentioned in it and we would then let it go if the max edits in
the file were less than the minimum edit currently carried by all regions (or if region had
moved -- we could ignore).  Maybe its not too bad.

Would it be worth doing?  It would scope-down sequenceid from being across the regionserver
to being within a region and would let us do some reasoning about edits per WAL.
                
> Mutations on Regions in recovery mode might have same sequenceIDs
> -----------------------------------------------------------------
>
>                 Key: HBASE-8741
>                 URL: https://issues.apache.org/jira/browse/HBASE-8741
>             Project: HBase
>          Issue Type: Bug
>          Components: MTTR
>    Affects Versions: 0.95.1
>            Reporter: Himanshu Vashishtha
>            Assignee: Himanshu Vashishtha
>
> Currently, when opening a region, we find the maximum sequence ID from all its HFiles
and then set the LogSequenceId of the log (in case the later is at a small value). This works
good in recovered.edits case as we are not writing to the region until we have replayed all
of its previous edits. 
> With distributed log replay, if we want to enable writes while a region is under recovery,
we need to make sure that the logSequenceId > maximum logSequenceId of the old regionserver.
Otherwise, we might have a situation where new edits have same (or smaller) sequenceIds. 
> We can store region level information in the WALTrailer, than this scenario could be
avoided by:
> a) reading the trailer of the "last completed" file, i.e., last wal file which has a
trailer and,
> b) completely reading the last wal file (this file would not have the trailer, so it
needs to be read completely).
> In future, if we switch to multi wal file, we could read the trailer for all completed
WAL files, and reading the remaining incomplete files.

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