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From "stack (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HBASE-2856) TestAcidGuarantee broken on trunk
Date Thu, 13 Jan 2011 05:12:46 GMT

    [ https://issues.apache.org/jira/browse/HBASE-2856?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12981132#action_12981132
] 

stack commented on HBASE-2856:
------------------------------

Thinking on the way home and after chatting more with Ryan, I'm thinking that we do not want
two sets of sequence numbers -- one at regionserver level (HLog sequence id) and then another
HRegion-scoped one.  All edits are bottlenecked via the WAL -- give or take the skip WAL write
flag and deferred flush -- and so arbitrarily breaking the transaction into two phases, the
WAL write then the commit to memstore, seems like it gains us little; edits will stack up
behind the WAL chicane anyways.

Paranoia regards performance issues come from long waits in 'wait to be visible' phase when
the Transaction spanned WAL.

I'm thinking now that for this issue, we do it first 'correct' -- e.g. use HLog sequence number
throughout and start the 'transaction' pre-WAL -- and then in a new issue figure the smarts
that will get us over the slowness we've seen before taking this tack.

> TestAcidGuarantee broken on trunk 
> ----------------------------------
>
>                 Key: HBASE-2856
>                 URL: https://issues.apache.org/jira/browse/HBASE-2856
>             Project: HBase
>          Issue Type: Bug
>    Affects Versions: 0.89.20100621
>            Reporter: ryan rawson
>            Assignee: stack
>            Priority: Blocker
>             Fix For: 0.92.0
>
>         Attachments: 2856-v2.txt, 2856-v3.txt, acid.txt
>
>
> TestAcidGuarantee has a test whereby it attempts to read a number of columns from a row,
and every so often the first column of N is different, when it should be the same.  This is
a bug deep inside the scanner whereby the first peek() of a row is done at time T then the
rest of the read is done at T+1 after a flush, thus the memstoreTS data is lost, and previously
'uncommitted' data becomes committed and flushed to disk.
> One possible solution is to introduce the memstoreTS (or similarly equivalent value)
to the HFile thus allowing us to preserve read consistency past flushes.  Another solution
involves fixing the scanners so that peek() is not destructive (and thus might return different
things at different times alas).

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