[HBase-issues] [jira] Updated: (HBASE-2248) Provide new non-copy mechanism to assure atomic reads in get and scan

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
Priority: Blocker
Fix For: 0.20.4

Attachments: HBASE-2248-demonstrate-previous-impl-bugs.patch, HBASE-2248-GetsAsScans3.patch, HBASE-2248-rr-alpha3.txt, HBASE-2248-rr-pre-durability.txt, HBASE-2248-rr-pre-durability2.txt, hbase-2248.gc, HBASE-2248.patch, hbase-2248.txt, readownwrites-lost.2.patch, readownwrites-lost.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.