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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 Wed, 19 Jun 2013 22:36:20 GMT

    [ https://issues.apache.org/jira/browse/HBASE-8741?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13688572#comment-13688572
] 

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

[~sershe] I think this a good scheme as long as we do stuff to make sure we do not block (go
get new batch of numbers before we hit the highest number allowed).  Having the data in a
system table would probably be best since we don't want to do some kind of negotiation among
regionservers to see what the highest number is on cluster restart.  It would be cluster-wide?
 What happens when the cluster hits Long.MAX_VALUE?

Can we come up w/ a scheme that doesn't have to have a cluster-wide number?  Couldn't we have
sequence numbers per region?

I thought Enis's scheme nice where you allow only N edits per file but realize now that it
does not work since sequenceid can be bumped if we get a region from a server whose sequenceid
count is way in excess of ours.

[~himanshu@cloudera.com] I like Sergey's proposal better.  Less moving parts.






                
> 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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