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# yum install epel-release -y
# yum update -y
# yum install vim tmux htop -y

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• Set up passwordless SSH between our nodes:

If passwordless ssh has not yet been set up within the cluster, then in main node generate key:

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# cat /dev/zero | ssh-keygen -q -N "" 2>/dev/null

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• If you're not installing on a single node, add this newly generated key to all the slave nodes:

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ssh-copy-id -i ~/.ssh/id_rsa.pub <replace_with_node_ip>

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Side note:

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You might have to adapt your sshd_config file and add "PermitRootLogin yes" amongst other parameters if you want passwordless root access, but that's outside the scope of this document.

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• Increase limits for ElasticSearch and Storm on nodes where you will be installing them (if you don't know, increase it everywhere):

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# echo -e "elasticsearch - memlock unlimited\nstorm - nproc 257597" >> /etc/security/limits.conf

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- Adjust limits to secure level ([link](https://docs.hortonworks.com/HDPDocuments/HDP2/HDP-2.3.0/bk_installing_manually_book/content/ref-729d1fb0-6d1b-459f-a18a-b5eba4540ab5.1.html)):
```
# ulimit -n 32768
# ulimit -u 65536
# echo -e "* - nofile 32768\n* - nproc 65536" >> /etc/security/limits.conf
```

- Disable IPv6, leaving it enabled may force service to bind to IPv6 addresses only and thus resulting in inability to connect to it ([source link](https://wiki.centos.org/FAQ/CentOS6#head-d47139912868bcb9d754441ecb6a8a10d41781df)):
Disable for the running system:
```
# sysctl -w net.ipv6.conf.all.disable_ipv6=1
# sysctl -w net.ipv6.conf.default.disable_ipv6=1
or
# echo 1 > /proc/sys/net/ipv6/conf/all/disable_ipv6
# echo 1 > /proc/sys/net/ipv6/conf/default/disable_ipv6
```
To survive a reboot:
Add:
net.ipv6.conf.all.disable_ipv6 = 1
net.ipv6.conf.default.disable_ipv6 = 1
To:
/etc/sysctl.conf
```
# echo -e "\n# Disable IPv6\nnet.ipv6.conf.all.disable_ipv6 = 1\nnet.ipv6.conf.default.disable_ipv6 = 1" >> /etc/sysctl.conf
```

- Disable Transparent Hugepage. Add "transparent_hugepage=never" to the end of the kernel line in /etc/default/grub and reboot. (Ambari demands it, do we need to comply?):
```
Change the line:
GRUB_CMDLINE_LINUX="crashkernel=auto rd.lvm.lv=cl/root rd.lvm.lv=cl/swap rhgb quiet"
To:
GRUB_CMDLINE_LINUX="crashkernel=auto rd.lvm.lv=cl/root rd.lvm.lv=cl/swap rhgb quiet
transparent_hugepage=never"
Afterwards, run:
# grub2-mkconfig -o /boot/grub2/grub.cfg
```
After reboot check that changes were applied (make sure that word "never" is selected in square-brackets):
```
# cat /sys/kernel/mm/transparent_hugepage/enabled
always madvise [never]
```

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