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From "stack (JIRA)" <j...@apache.org>
Subject [jira] Updated: (HBASE-588) Still a 'hole' in scanners, even after HBASE-532
Date Fri, 18 Apr 2008 15:56:21 GMT

     [ https://issues.apache.org/jira/browse/HBASE-588?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel

stack updated HBASE-588:

    Fix Version/s: 0.1.2

The Jim soln. means keeping around possibly two Maps where one is at least 64M per columnfamily,
per region while a scanner is outstanding.

Making this issue a fix for 0.1.2.

> Still a 'hole' in scanners, even after HBASE-532
> ------------------------------------------------
>                 Key: HBASE-588
>                 URL: https://issues.apache.org/jira/browse/HBASE-588
>             Project: Hadoop HBase
>          Issue Type: Bug
>            Reporter: stack
>            Priority: Blocker
>             Fix For: 0.1.2
> Before HBASE-532, as soon as a flush started, we called snapshot.  Snapshot used to copy
current live memcache into a 'snapshot' TreeMap inside in Memcache.  This snapshot TreeMap
was an accumulation of all snapshots since last flush.   Whenever we took out a scanner, we'd
do a copy of this snapshot into a new backing map carried by the scanner (Every outstanding
Scanner had complete copy).  Memcache snapshots were cleared when a flush started.   Flushing
could take near no time to up to tens of seconds during which an scanners taken out meantime
would not see the edits in the snapshot currently being flushed and gets or getFull would
also return incorrect answers because the content of the snapshot was not available to them.
> HBASE-532 made it so the snapshot was available until flush was done -- until a file
had made it out to disk.  This fixed gets and getFull and any scanners taken out during flushing.
 But there is still a hole.  Any outstanding scanners will be going against the state of Store
Readers at time scanner was opened; they will not see the new flush file.
> Chatting about this on IRC, Jim suggests that we pass either memcache or current snapshot
to each Scanner (Pass the snapshot if not empty).  The notion is that the Scanner would hold
on to the Scanner reference should it be cleared by flushing.  Upside is that scanner wouldn't
have to be concerned with the new flush that has been put out to disk.  Downsides are that
Scanner data could be way stale if for instance the memcache was near to flushing but we hadn't
done it yet.  And we wouldn't be clearing the snapshot promptly so would be some memory pressure.
> Another suggestion is that flushing send an event.  Listeners such as outstanding scanners
would notice event and open the new Reader.  Would have to skip forward in the new Reader
to catch up with the current set but shouldn't be bad.  Same mechanism could be used to let
compactions be moved into place while scanners were outstanding closing down all existing
readers skipping to the current 'next' location in the new compacted store file.

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