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https://issues.apache.org/jira/browse/HBASE-2248?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12839964#action_12839964
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ryan rawson commented on HBASE-2248:
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I'm working on this, there is a general approach hammered out and code to be
written.
The approach is like so:
- on read from memstore, for each row, we grab the 'read number' and ignore any
keyvalues in the structure newer (ie: > value)
- on put to hregion/memstore, we start a write 'tx' and get a write-number, and
put keyvalues with said write-number. when we are finished, that write-number
is 'commited' which causes the read number to be advanced most of the time.
under concurrent writes we have a little queue and slower puts may slightly
hold up puts that come before it.
this will need to be extensively tested to see how the performance profile
changes. it will allow us to remove the newScannerLock.
> Provide new non-copy mechanism to assure atomic reads in get and scan
> ---------------------------------------------------------------------
>
> Key: HBASE-2248
> URL: https://issues.apache.org/jira/browse/HBASE-2248
> Project: Hadoop HBase
> Issue Type: Bug
> Affects Versions: 0.20.3
> Reporter: Dave Latham
> Fix For: 0.20.4
>
> Attachments: HBASE-2248-demonstrate-previous-impl-bugs.patch,
> hbase-2248.gc, HBASE-2248.patch, Screen shot 2010-02-23 at 10.33.38 AM.png,
> threads.txt
>
>
> HBASE-2037 introduced a new MemStoreScanner which triggers a
> ConcurrentSkipListMap.buildFromSorted clone of the memstore and snapshot when
> starting a scan.
> After upgrading to 0.20.3, we noticed a big slowdown in our use of short
> scans. Some of our data repesent a time series. The data is stored in time
> series order, MR jobs often insert/update new data at the end of the series,
> and queries usually have to pick up some or all of the series. These are
> often scans of 0-100 rows at a time. To load one page, we'll observe about
> 20 such scans being triggered concurrently, and they take 2 seconds to
> complete. Doing a thread dump of a region server shows many threads in
> ConcurrentSkipListMap.biuldFromSorted which traverses the entire map of key
> values to copy it.
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