Upgrading Red Hat Enterprise Linux / CentOS / Oracle Linux

RHEL/CentOS/Oracle Linux Migration: Storage

The default file system has changed multiple times, going from Ext3 to Ext4 to XFS, and possibly on to Btrfs in some distributions. Given a recent enough kernel and user-space tools, you could have used later file systems in some releases, and you still have full backward-compatable support of all older file systems. The file /etc/fstab has become less complex with the kernel automatically mounting many of the pseudo file systems without them being explicitly listed.

The previous page described the relatively minor changes in user and group management. Now that we have created these user accounts, we must set up storage for all their data!

5 — Storage

The default file system changed twice in going from RHEL 5 (Ext3) to RHEL 6 (Ext4) to RHEL 7 (XFS). It initially surprised me when Red Hat dropped Btrfs support, but then I realized that it's Oracle's product.

Oracle bought Sun Microsystems in January 2010, and soon retired both the Solaris operating system and the SPARC hardware on which it ran. I think that Oracle bought Sun to get the ZFS file system. They have been porting ZFS features into Btrfs. The head of the Ext4 file system project, an Oracle employee, said that Ext4 was just a temporary stepping stone on the way to Btrfs.

Meanwhile, Red Hat deprecated Btrfs at RHEL 7.4. My new theory is that Red Hat is anti-Btrfs at least partly because Btrfs may soon be seen as Oracle's file system.

RHEL 8 added the Stratis local storage manager.

The kernel drivers used for IDE/PATA disks have changed. IDE/PATA disks appeared as devices /dev/hd? in RHEL 5, starting with RHEL 6 all disks are /dev/sd?.

File system device definitions in /etc/fstab were in terms of LABEL= in RHEL 5. That changed to UUID= in RHEL 6.

Starting with RHEL 7, it is no longer practical for /usr to be a separate file system. This isn't specifically the fault of Red Hat's design. The components used by all distributions now disregard or violate the classic UNIX convention that the root file system be small with just what's needed for booting, while /usr has things needed later. But now /bin, /lib, /lib64 and /sbin are just symbolic links to directories within /usr.

Support for exFAT file systems, used by default in newer flash memory, was finally added at version 9.

Red Hat's recommendation for swap area continues to change and contradict itself. For example, install RHEL 7 onto a system with 8 GB of RAM and let the installer set up the storage. Red Hat's documentation recommends either 4 or 8 GB in that situation, but their installer creates a 2 GB swap area for RHEL 7. RHEL 8 on that same platform creates an 8 GB swap area.

It's time to kill off the old assumptions about Oracle needing unusual amounts of swap and a raw partition of its own. Put Oracle on a modern system with plenty of RAM and an efficient file system.

The NFS server now uses /etc/exports.d/* in addition to /etc/exports.

The automounter now uses /etc/auto.master.d/* in addition to /etc/auto.master.

Starting with RHEL 7, the mount command shows a larger list of mounts that aren't file systems appearing in routine df output. They do appear if you run df -a.

However, at least the /etc/fstab file is much simpler in RHEL 7 and 8.

If you have been using LUKS for encrypted file systems, RHEL 8 uses LUKS2 by default, versus legacy LUKS (LUKS1) used in earlier versions.

These lines were in /etc/fstab in RHEL 5 and 6. The lines are gone in RHEL 7. The file systems are now mounted automatically by the kernel, they don't clutter up /etc/fstab:

none  /dev/pts  devpts  gid=5,mode=620  0 0
none  /dev/shm  tmpfs   defaults        0 0
none  /proc     proc    defaults        0 0
none  /sys      sysfs   defaults        0 0