Gopal V created HDFS-4710: ----------------------------- Summary: Turning off HDFS short-circuit checksums unexpectedly slows down Hive Key: HDFS-4710 URL: https://issues.apache.org/jira/browse/HDFS-4710 Project: Hadoop HDFS Issue Type: Bug Components: hdfs-client Affects Versions: 2.0.4-alpha Environment: Centos (EC2) + short-circuit reads on Reporter: Gopal V Priority: Minor
When short-circuit reads are on, HDFS client slows down when checksums are turned off. With checksums on, the query takes 45.341 seconds and with it turned off, it takes 56.345 seconds. This is slower than the speeds observed when short-circuiting is turned off. The issue seems to be that FSDataInputStream.readByte() calls are directly transferred to the disk fd when the checksums are turned off. Even though all the columns are integers, the data being read will be read via DataInputStream which does {code} public final int readInt() throws IOException { int ch1 = in.read(); int ch2 = in.read(); int ch3 = in.read(); int ch4 = in.read(); {code} To confirm, an strace of the Yarn container shows {code} 26690 read(154, "B", 1) = 1 26690 read(154, "\250", 1) = 1 26690 read(154, ".", 1) = 1 26690 read(154, "\24", 1) = 1 {code} To emulate this without the entirety of Hive code, I have written a simpler test app https://github.com/t3rmin4t0r/shortcircuit-reader The jar will read a file in -bs <n> sized buffers. Running it with 1 byte blocks gives similar results to the Hive test run. -- This message is automatically generated by JIRA. If you think it was sent incorrectly, please contact your JIRA administrators For more information on JIRA, see: http://www.atlassian.com/software/jira