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From "stack (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-10241) implement mvcc-consistent scanners (across recovery)
Date Fri, 27 Dec 2013 17:05:52 GMT

    [ https://issues.apache.org/jira/browse/HBASE-10241?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13857563#comment-13857563

stack commented on HBASE-10241:

[~sershe] Nice writeup.

sequenceid is not scoped to the region. Ergo mvcc should be too?

bq. Client will have a configurable setting to enable consistent scanners.

IMO, just say no to more configs.  When would someone want 'inconsistent scanners'?  (In my
experience, users do not play with configs in 98.1745% of cases -- smile).

bq. ....unless mvcc and seqId are merged)

Would this issue be better if above work is done first?

bq. ...Recovery will have to make use of mvcc when replaying the edit to new server.

This work is ongoing over in @jeffrey zhong replay wal effort?

mvcc is about giving you a consistent view on a row only.  This work is to deal with the case
where you have a wide row and you have already passed the client the first half of a row,
a crash happens, and you need to return to the client the second half of the row?

A section in doc. on implications of not having this change fixed and then the 'cost' of this
fix going in would help.

Thanks for working on this tough one [~sershe]

[~lhofhansl] One for you to peruse boss...

> implement mvcc-consistent scanners (across recovery)
> ----------------------------------------------------
>                 Key: HBASE-10241
>                 URL: https://issues.apache.org/jira/browse/HBASE-10241
>             Project: HBase
>          Issue Type: New Feature
>          Components: HFile, regionserver, Scanners
>    Affects Versions: 0.99.0
>            Reporter: Sergey Shelukhin
>            Assignee: Sergey Shelukhin
>         Attachments: Consistent scanners.pdf
> Scanners currently use mvcc for consistency. However, mvcc is lost on server restart,
or even a region move. This JIRA is to enable the scanners to transfer mvcc (or seqId, or
some other number, see HBASE-8763) between servers. First, client scanner needs to get and
store the readpoint. Second, mvcc needs to be preserved in WAL. Third, the mvcc needs to be
stored in store files per KV and discarded when not needed.

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