Features supported by different releases of hadoop

Some optimization trics for hadoop & mapreduce

Here are the some parameters which we can use to optimize and utilize hadoop and maprduce in a bit better way.

these parameter and their values are not fixed and the optimization and different parameter test must be done to optimize closely Hadoop according to the set up and type of machines in the cluster.

io.sort.factor-->64

io.sort.mb-->254

Mapred.reduce.parallel.copies

-->(number of machines*number of mappers)/2 (generally)

mapred.tasktracker.(map|reduce).task.maximum

-->map less than cores(if8cores then 5-10)  (generally)

-->reduce(less than mapper, 4-6-8) (generally)

-->number of map+reduce>number of cores (generally)

mapred.(map|reduce).task.speculative.execution-->true

-->Same task to be executed on more than one machine in parallel

Tasktracker.http.threads

-->HTTP threads should be enough to support parallel copies in sort and snuffle phase.

We can use LZO compressed.

Use combiner

Impliment a custom partioner 

Input split 

~64-128-256(size of each file or block)

MySQL: SUBSTRING_INDEX - Select Patterns

Consider, a MySQL table having values in a column like below:

SELECT location  FROM geo LIMIT 3;

"location"
"India.Karnataka.Shimoga.Gopala"
"India.Karnataka.Bengaluru.BTM"
"India.Karnataka.Chikmaglore.Koppa"

My requirement is to take only 4th value from each of the rows(such as, Gopala,BTM,Koppa). 

I don't want to display remaining values. 

Its same as what 'cut' command will do in Linux.

For this, we can use SUBSTRING_INDEX function.

SELECT SUBSTRING_INDEX(location,'.',-1)  from geo LIMIT 3;

"location"

"Gopala"

"BTM"

"Koppa"

Syntax: SUBSTRING_INDEX(string,delimiter,count)

Here count means column number based on delimiter. 

Negative value indicates that the column numbers calculated from right side.

So, if I give '-2' instead of  '-1':

SELECT SUBSTRING_INDEX(location,'.',-2)  from geo LIMIT 3;

"location"

"Shimoga.Gopala"

"Bengaluru.BTM"

"Chikmaglore.Koppa"

Hadoop over utilization of Hdfs

Do you face problem of over uses of HDFS for datanode, frequently it becomes 100% and hence result in a imbalance cluster, thinking of how to solve this problem, for this what we can do is put a parameter called "dfs.datanode.du.reserved" so this will reserve the non HDFS uses disk space and hence leaving the some space remaining for non HDFS uses and solving disk overuses of HDFS .

Hadoop small file and block allocation

One of the misconceptions about Hadoop is that smaller files (smaller than the block size 64 MB default) will still use the whole block on the filesystem and there will be space westage on hdfs. This is not the true in reality. The smaller files occupy exactly as much disk space as they require(1 mb file at local disk will somwhat same space on hdfs). But this does not mean that having many small files will use HDFS efficiently. Regardless of the block size, its metadata at namenode occupies exactly the same amount of memory. As a result, a large number of small HDFS files (smaller than the block size) will use a lot of the NameNode’s memory, thus negatively impacting HDFS scalability and performance.

So HDFS blocks are not a storage allocation unit, but a replication unit.

List of different hadoop distribution.

Cloudera CDH,Manager, and Enterprise

Based on Hadoop 2, CDH (version 4.1.2 as of this writing) includes HDFS, YARN, HBase, MapReduce, Hive, Pig, Zookeeper, Oozie, Mahout, Hue, and other open source tools (including the real-time query engine — Impala).

Hortonworks Data Platform

Incriment variable in linux shell

Following are the listed methods by which we can increment the variables in shell script in looping statements:

• j=$((i++))

• j=$(( i + 1 ))

• j=`expr $i + 1`

Happy scripting :)

Get file permission in numeric form in linux

We can use the following command to get the file permission in numeric form instead of string from.

stat -c "%a %n" *

Where 

-c   --> specify custom format

%a --> Access rights in octal format

%n --> file name

MySQL: Replication - Skip sql statement from execution

In any MySQL Replication set up, some or the other time we come across errors on the Slave due to which, replication stops.  

If the error is due to manual intervention on replication, we may have to skip the current SQL transaction on Slave which caused the error.

In the case, below is the command to be executed:

SET GLOBAL sql_slave_skip_counter=1;
START SLAVE;


You can increase the counter value if you want to skip multiple consecutive transactions.

Prevent accidental data loss in Hadoop

Some time you have accidentally delete some file which you are not suppose to do. So what will you do in that case? 

The option with you is to enable trash (Recycle bin) for hadoop, and define fs.trash.interval 

to one day with 1440 Minutes or more or less according to the cost of data you host in hadoop.

You can do it in following way :

Open core-site.xml and paste following parameter 

  

Display two files side by side in linux

Have you ever needed to display cat content of two files side by side on terminal, if yes you are right place :)

You have following options :