Upgrading Red Hat Enterprise Linux / CentOS

RHEL/CentOS 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.

Previous: Users and Groups,
Identity Management

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). The big surprise to me is that it did not change again going to RHEL 8!

Release	Default FS	Maximum
Release	Default FS	Volume size	File size
RHEL 5	Ext3	4–32 TiB	16 GiB – 2 TiB
RHEL 6	Ext4	1 EiB	16 TiB
RHEL 7	XFS	16 EiB	8 EiB
RHEL 8β	XFS	16 EiB	8 EiB
?	Btrfs	16 EiB	8 EiB

1 TiB = 2⁴⁰ bytes = 1024 GiB
1 PiB = 2⁵⁰ bytes = 1024 TiB
1 EiB = 2⁶⁰ bytes = 1024 PiB

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

But then RHEL 8β came out with XFS by default. You can choose Btrfs, as with RHEL 7, but XFS remains the default.

My new theory is that Oracle wanted the Sun support contracts, and they weren't really looking to buy either the hardware or software...

Release	Default FS	File System Tools
Release	Default FS	Create	Modify/view metadata	Modify/grow file system	Check/Repair	Dump Restore
RHEL 5	Ext3	`mkfs.ext3`	`tune2fs dumpe2fs`	`resize2fs`	`fsck.ext3`	`dump restore`
RHEL 6	Ext4	`mkfs.ext4`	`tune2fs dumpe2fs`	`resize2fs`	`fsck.ext4`	`dump restore`
RHEL 7	XFS	`mkfs.xfs`	`xfs_admin`	`xfs_growfs`	`xfs_repair`	`xfsdump xfsrestore`
RHEL 8β	XFS	`mkfs.xfs`	`xfs_admin`	`xfs_growfs`	`xfs_repair`	`xfsdump xfsrestore`
?	Btrfs	`mkfs.btrfs`	`btrfs filesystem` `btrfs-debug-tree` `btrfs property`	`btrfs filesystem`	`btrfs scrub btrfsck btrfs rescue btrfs restore`	`btrfs image btrfs image`

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 old 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.

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. With RHEL 8β, it 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.

The mount command shows an ever-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.

Release	Physical RAM	Swap Recommendation	To allow hibernation
RHEL 4	always	at least 32 MB
	≤ 2 GB	2 × RAM
	> 2 GB	2 GB + RAM
RHEL 5	< 4 GB	at least 2 GB
	4–16 GB	4 GB
	16–64 GB	8 GB
	64–256 GB	16 GB
	256–512 GB	32 GB
RHEL 6	≤ 2 GB	2 × RAM	3 × RAM
	2–8 GB	same as RAM	2 × RAM
	8–64 GB	at least 4 GB	1.5 × RAM
	> 64 GB	at least 4 GB	don't
RHEL 7	≤ 2 GB	2 × RAM	3 × RAM
	2–8 GB	same as RAM	2 × RAM
	8–64 GB	0.5 × RAM	1.5 × RAM
	> 64 GB	it depends	don't

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

Now that we can add storage as needed, we can start some large software development projects. The next page combines two related topics: package management with rpm and yum (and, later, dnf), and building software from source.

Next ❯ Software Packages