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HBase >> mail # user >> Strange behavior on scan while writing

Placido Revilla 2011-10-03, 10:52
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Re: Strange behavior on scan while writing
Sorry, resent because I messed up the previous mail.


we are experiencing a strange behavior in some tests we are currently
performing. What we are seeing is that scans on a table that is being
written to at the same time sometimes end prematurely, with no error.
This seems to be heavily dependent on the write pattern.

We've been able to reproduce the issue with the standard hbase tools:

in a terminal run:

OLDV=0; OLDT=0; while true; do NEWV=`hbase shell count_testtable |
head -1 | cut -d' ' -f1`; NEWT=`date +%s`; echo $NEWV " -> " $(((NEWV
- OLDV) / (NEWT - OLDT))) "msg/s"; OLDV=$NEWV; OLDT=$NEWT; done

where the contents of the file count_testtable are:

count 'TestTable', INTERVAL => 100000000, CACHE => 10000

This counts the rows in the TestTable repeatedly showing the number of
rows and the delta rows per second inserted. In the hbase shell count
is implemented as a full scan with a filter on the row key.

Meanwhile, in another terminal do:

hbase org.apache.hadoop.hbase.PerformanceEvaluation --nomapred
--rows=20000 randomWrite 5

and when over:

hbase org.apache.hadoop.hbase.PerformanceEvaluation --nomapred
--rows=20000 sequentialWrite 5

On the scan terminal we are seeing results similar to:

0  ->  0 msg/s
45552  ->  5694 msg/s <=== randomWrite starts
63284  ->  2955 msg/s
63284  ->  0 msg/s  <=== randomWrite ends
58829  ->  -636 msg/s <=== sequentialWrite starts
88764  ->  3741 msg/s
100000  ->  802 msg/s
100000  ->  0 msg/s <=== sequentialWrite ends

As you can see in the fifth row the count is lower than expected
(resulting on a negative inserts/sec).

You may need to try a couple of times or tweak the number of rows to
insert to see the problem.

hbase version: 0.90.4 (tried on a standalone and a full distributed deployment).

We think there must be an error somewhere or something we don't
understand is slipping by us.


Plácido Revilla
Senior Backend Engineer
Jean-Daniel Cryans 2011-10-04, 22:35
Placido Revilla 2011-10-05, 07:41
Lars 2011-10-03, 16:11
Placido Revilla 2011-10-04, 09:57
lars hofhansl 2011-10-05, 03:57