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From "Jonathan Hsieh (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-6055) Snapshots in HBase 0.96
Date Mon, 28 May 2012 19:39:25 GMT

    [ https://issues.apache.org/jira/browse/HBASE-6055?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13284532#comment-13284532

Jonathan Hsieh commented on HBASE-6055:


Thanks for the writeup, I find having a single doc with the design summary really helpful
and ideally something we do for all major new features. I've read through the document carefully,
let it steep for a few days, and had some design-level questions.  I've skimmed HBASE-50 and
will read more of the history more carefully later this evening.  

What is the read mechanism for snapshots like?  Does the snapshot act like a read-only table
or is there some special external mechanism needed to read the data from a snapshot?  You
mention having to rebuild in-memory state by replaying wals -- is this a recovery situation
or needed in normal reads?

What is a representation of a snapshot look like in terms of META and file system contents?
 At some point we may get called upon to repair these, I want to make sure there are enough
breadcrumbs for this to be possible.

I'm still thinking about the two-phase part -- I think it is necessary for marking success
or initiating failure recovery, but I'm skeptical at the moment about why the barriering writes
is necessary.  How does this buy your more consistency?  Aren't we still inconsistent at the
prepare point now instead?   Can we just write the special snapshotting hlog entry at initiation
of prepare, allowing writes to continue, then adding data elsewhere (META) to mark success
in commit?  We could then have some compaction/flush time logic cleanup failed atttempt markers?

> Snapshots in HBase 0.96
> -----------------------
>                 Key: HBASE-6055
>                 URL: https://issues.apache.org/jira/browse/HBASE-6055
>             Project: HBase
>          Issue Type: New Feature
>          Components: client, master, regionserver, zookeeper
>            Reporter: Jesse Yates
>            Assignee: Jesse Yates
>             Fix For: 0.96.0
>         Attachments: Snapshots in HBase.docx
> Continuation of HBASE-50 for the current trunk. Since the implementation has drastically
changed, opening as a new ticket.

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