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From "Todd Lipcon (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HBASE-2312) Possible data loss when RS goes into GC pause while rolling HLog
Date Mon, 15 Mar 2010 21:15:27 GMT

    [ https://issues.apache.org/jira/browse/HBASE-2312?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12845531#action_12845531
] 

Todd Lipcon commented on HBASE-2312:
------------------------------------

Yea, like I said I think it's really unlikely. What I'm thinking is something like:

|*RS*|*Master*|
|Writes "intent to roll to hlog.2"|-|
|enters GC pause|-|
|-|detects RS dead, lists files, sees only up to hlog.1|
|comes back to life|-|
|opens hlog.2 for append|-|
|writes log.1 rolled, closes|-|
|-|sees intent to roll at end of hlog.1|
|writes some edits to hlog.2|-|
|writes "intent to roll to hlog.3"|-|
|opens hlog.3 for append|-|
|writes "rolled" to hlog.2, closes|-|
|-|opens hlog.2 for append|
|-|*needs to check for intent to roll at end of hlog.2*|


Like I said this is very unlikely :)

> Possible data loss when RS goes into GC pause while rolling HLog
> ----------------------------------------------------------------
>
>                 Key: HBASE-2312
>                 URL: https://issues.apache.org/jira/browse/HBASE-2312
>             Project: Hadoop HBase
>          Issue Type: Bug
>          Components: master, regionserver
>    Affects Versions: 0.20.3
>            Reporter: Karthik Ranganathan
>
> There is a very corner case when bad things could happen(ie data loss):
> 1)	RS #1 is going to roll its HLog - not yet created the new one, old one will get no
more writes
> 2)	RS #1 enters GC Pause of Death
> 3)	Master lists HLog files of RS#1 that is has to split as RS#1 is dead, starts splitting
> 4)	RS #1 wakes up, created the new HLog (previous one was rolled) and appends an edit
- which is lost
> The following seems like a possible solution:
> 1)	Master detects RS#1 is dead
> 2)	The master renames the /hbase/.logs/<regionserver name>  directory to something
else (say /hbase/.logs/<regionserver name>-dead)
> 3)	Add mkdir support (as opposed to mkdirs) to HDFS - so that a file create fails if
the directory doesn't exist. Dhruba tells me this is very doable.
> 4)	RS#1 comes back up and is not able create the new hlog. It restarts itself.

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