[HBASE-2248] Provide new non-copy mechanism to assure atomic reads in get and scan

• Type: Bug
• Status: Closed
• Priority: Blocker
• Resolution: Fixed
• Affects Version/s: 0.20.3
• Fix Version/s: 0.20.4
• Hadoop Flags: Incompatible change, Reviewed

Release Note

This patch changes the Get code path to instead be a Scan of one row. This means than inserting cells out of timestamp order should work now (tests to verify to follow part of hbase-2294) but also that a delete at an explicit timestamp now overshadows EVEN if the effected cell is put after the delete (The old Get code path did early-out so a subsequent puts would not see the delete).

• Assignee: ryan rawson
• Reporter: Dave Latham
• Created: 22/Feb/10 23:35
• Updated: 12/Oct/12 06:14
• Resolved: 14/Apr/10 21:57

Description

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.

Issue Links

• is related to: HBASE-2959 Scanning always starts at the beginning of a row
• is related to: HBASE-2294 Enumerate ACID properties of HBase in a well defined spec
• relates to: HBASE-2322 deadlock between put and cacheflusher in 0.20 branch
• relates to: HBASE-2249 The PerformanceEvaluation read tests don't take the MemStore into account.
• relates to: HBASE-2265 HFile and Memstore should maintain minimum and maximum timestamps