what is the difference between rhel 6 and rhel 7 ?

Features	RHEL 7	RHEL 6
Default File System	XFS	EXT4
Kernel Version	3.10.x-x kernel	2.6.x-x Kernel
Kernel Code Name	Maipo	Santiago
General Availability Date of First Major Release	2014-06-09 (Kernel Version 3.10.0-123)	2010-11-09 (Kernel Version 2.6.32-71)
First Process	systemd (process ID 1)	init (process ID 1)
Runlevel	runlevels are called as "targets" as shown below: runlevel0.target -> poweroff.target runlevel1.target -> rescue.target runlevel2.target -> multi-user.target runlevel3.target -> multi-user.target runlevel4.target -> multi-user.target runlevel5.target -> graphical.target runlevel6.target -> reboot.target /etc/systemd/system/default.target (this by default is linked to the multi-user target)	Traditional runlevels defined : runlevel 0 runlevel 1 runlevel 2 runlevel 3 runlevel 4 runlevel 5 runlevel 6 and the default runlevel would be defined in /etc/inittab file. /etc/inittab
Host Name Change	In Red Hat Enterprise Linux 7, as part of the move to the new init system (systemd), the hostname variable is defined in /etc/hostname.	In Red Hat Enterprise Linux 6, the hostname variable was defined in the /etc/sysconfig/network configuration file.
Change In UID Allocation	By default any new users created would get UIDs assigned starting from 1000. This could be changed in /etc/login.defs if required.	Default UID assigned to users would start from 500. This could be changed in /etc/login.defs if required.
Max Supported File Size	Maximum (individual) file size = 500TB Maximum filesystem size = 500TB (This maximum file size is only on 64-bit machines. Red Hat Enterprise Linux does not support XFS on 32-bit machines.)	Maximum (individual) file size = 16TB Maximum filesystem size = 16TB (This maximum file size is based on a 64-bit machine. On a 32-bit machine, the maximum files size is 8TB.)
File System Check	"xfs_repair" XFS does not run a file system check at boot time.	"e2fsck" File system check would gets executed at boot time.
Differences Between xfs_repair & e2fsck	"xfs_repair" - Inode and inode blockmap (addressing) checks. - Inode allocation map checks. - Inode size checks. - Directory checks. - Pathname checks. - Link count checks. - Freemap checks. - Super block checks.	"e2fsck" - Inode, block, and size checks. - Directory structure checks. - Directory connectivity checks. - Reference count checks. - Group summary info checks.
Difference Between xfs_growfs & resize2fs	"xfs_growfs" xfs_growfs takes mount point as arguments.	"resize2fs" resize2fs takes logical volume name as arguments.
Change In File System Structure	/bin, /sbin, /lib, and /lib64 are now nested under /usr.	/bin, /sbin, /lib, and /lib64 are usually under /
Boot Loader	GRUB 2 Supports GPT, additional firmware types, including BIOS, EFI and OpenFirmwar. Ability to boot on various file systems (xfs, ext4, ntfs, hfs+, raid, etc)	GRUB 0.97
KDUMP	RHEL7 supports kdump on large memory based systems up to 3 TB	Kdump doesn’t work properly with large RAM based systems.
System & Service Manager	"Systemd" systemd is a system and service manager for Linux, and replaces SysV and Upstart used in previous releases of Red Hat Enterprise Linux. systemd is compatible with SysV and Linux Standard Base init scripts.	Upstart
Enable/Start Service	For RHEL 7, the systemctl command replaces service and chkconfig. - Start Service : "systemctl start nfs-server.service". - Enable Service : To enable the service (example: nfs service ) to start automatically on boot : "systemctl enable nfs-server.service". Although one can still use the service and chkconfig commands to start/stop and enable/disable services, respectively, they are not 100% compatible with the RHEL 7 systemctl command (according to redhat).	Using "service" command and "chkconfig" commands. - Start Service : "service start nfs" OR "/etc/init.d/nfs start" - Enable Service : To start with specific runlevel : "chkconfig --level 3 5 nfs on"
Default Firewall	"Firewalld (Dynamic Firewall)" The built-in configuration is located under the /usr/lib/firewalld directory. The configuration that you can customize is under the /etc/firewalld directory. It is not possible to use Firewalld and Iptables at the same time. But it is still possible to disable Firewalld and use Iptables as before.	Iptables
Network Bonding	"Team Driver" -/etc/sysconfig/network-scripts/ifcfg-team0 - DEVICE=”team0” - DEVICETYPE=”Team”	"Bonding" -/etc/sysconfig/network-scripts/ifcfg-bond0 - DEVICE=”bond0”
Network Time Synchronization	Using Chrony suite (faster time sync compared with ntpd)	Using ntpd
NFS	NFS4.1 NFSv2 is no longer supported. Red Hat Enterprise Linux 7 supports NFSv3, NFSv4.0, and NVSv4.1 clients.	NFS4
Cluster Resource Manager	Pacemaker	Rgmanager
Load Balancer Technology	Keepalived and HAProxy	Piranha
Desktop/GUI Interface	GNOME3 and KDE 4.10	GNOME2
Default Database	MariaDB is the default implementation of MySQL in Red Hat Enterprise Linux 7	MySQL
Managing Temporary Files	RHEL 7 uses systemd-tmpfiles (more structured, and configurable, method to manage tmp files and directories).	Using "tmpwatch"