[jira] Commented: (HADOOP-1903) [hbase] Data loss if Exception happens between snapshot and flush to disk.

["stack (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/HADOOP-1903?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12527687 ] 

stack commented on HADOOP-1903:
-------------------------------

On second take, if the interruption causes a regionserver crash, there should be no data loss; memaches is backed upin the hlog.  But if we manage to keep going, then we'll be in a strange situation where the snapshot has been 'lost' -- though backed up in hlog -- because regionserver serves out of memcache+snapshot histories and from flushes, the store HStoreFiles.  

> [hbase] Data loss if Exception happens between snapshot and flush to disk.
> --------------------------------------------------------------------------
>
>                 Key: HADOOP-1903
>                 URL: https://issues.apache.org/jira/browse/HADOOP-1903
>             Project: Hadoop
>          Issue Type: Bug
>            Reporter: stack
>
> There exists a little window during which we can lose data.  During a memcache flush, we make an inmemory copy, a 'snapshot'.  The memcache is then zeroed and off we go again taking updates.  Meantime, in background we are supposed to flush the snapshot to disk.  If this process is interrupted -- e.g. the HDFS is yanked from under us or if an OOME occurs in this thread -- then the content of the snapshot is lost.

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