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dcacheck gives a WARNING message for the "blockdev readahead"

Problem

Running a dcacheck gives a WARNING message for the "blockdev readahead".

   20140714:10:33:06:024863 dcacheck_main.py:mdw:root-[ERROR]:-DCA_CHECK_ERROR host(sdw16): on device (/dev/sdb) blockdev readahead value '256' does not match expected value '16384'

Solution

Each disk device file should have a read-ahead (blockdev) value of 16384. If there is a disk devise with a different value then this will need to be changed. To check the currently value across the cluster

# gpssh -f /home/gpadmin/gpconfigs/hostfile_segments
=> /sbin/blockdev --getra /dev/sdb
[sdw1] 16384
[sdw2] 256
[sdw3] 16384
[sdw4] 16384
[sdw5] 16384
[sdw6] 16384
[sdw7] 16384
[sdw8] 16384

To set the device on the particular server that has a different value, you will need to connect to the server as the root user and then run the following command for the particular device that needs to be changed:

    # /sbin/blockdev --setra 16384 /dev/sdb

NOTE: 

  • This should be done when the DB is stopped, then run dcacheck again to ensure the WARNING message is gone.
  • The Read performance of a Disk can be improved by increasing a parameter called "Read+Ahead" using 'blockdev' command. By default the Linux OS will read 128 KB of data in advance so that it is already in Memory cache before the program needs it. This value can be increased so as to get better Read Performance. In Greenplum the recommended values for this is 16384(8MB).

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