Chapter 21. File Systems

OverlayFS is a type of union file system. It allows the user to overlay one file system on top of another. Changes are recorded in the upper file system, while the lower file system remains unmodified. This allows multiple users to share a file-system image, such as a container or a DVD-ROM, where the base image is on read-only media. Refer to the kernel file Documentation/filesystems/overlayfs.txt for additional information.

OverlayFS remains a Technology Preview in Red Hat Enterprise Linux 7.2 under most circumstances. As such, the kernel will log warnings when this technology is activated.

Full support is available for OverlayFS when used with Docker under the following restrictions:

* OverlayFS is only supported for use as a Docker graph driver. Its use can only be supported for container COW content, not for persistent storage. Any persistent storage must be placed on non-OverlayFS volumes to be supported. Only default Docker configuration can be used; that is, one level of overlay, one lowerdir, and both lower and upper levels are on the same file system.

* Only XFS is currently supported for use as a lower layer file system.

* SELinux must be enabled and in enforcing mode on the physical machine, but must be disabled in the container when performing container separation; that is, /etc/sysconfig/docker must not contain --selinux-enabled. SELinux support for OverlayFS is being worked on upstream, and is expected in a future release.

* The OverlayFS kernel ABI and userspace behavior are not considered stable, and may see changes in future updates.

* In order to make the yum and rpm utilities work properly inside the container, the user should be using the yum-plugin-ovl packages.

Note that OverlayFS provides a restricted set of the POSIX standards. Test your application thoroughly before deploying it with OverlayFS.

Note that XFS file systems must be created with the -n ftype=1 option enabled for use as an overlay. With the rootfs and any file systems created during system installation, set the --mkfsoptions=-n ftype=1 parameters in the Anaconda kickstart. When creating a new file system after the installation, run the # mkfs -t xfs -n ftype=1 /PATH/TO/DEVICE command. To determine whether an existing file system is eligible for use as an overlay, run the # xfs_info /PATH/TO/DEVICE | grep ftype command to see if the ftype=1 option is enabled.

There are also several known issues associated with OverlayFS as of Red Hat Enterprise Linux 7.2 release. For details, see 'Non-standard behavior' in the Documentation/filesystems/overlayfs.txt file.

Red Hat Enterprise Linux 7.2 adds support for flexible file layout on NFSv4 clients. This technology enables advanced features such as non-disruptive file mobility and client-side mirroring, providing enhanced usability in areas such as databases, big data and virtualization.

See https://datatracker.ietf.org/doc/draft-ietf-nfsv4-flex-files/ for detailed information about NFS flexible file layout.

The Btrfs (B-Tree) file system is supported as a Technology Preview in Red Hat Enterprise Linux 7.2. This file system offers advanced management, reliability, and scalability features. It enables users to create snapshots, it enables compression and integrated device management.

As a Technology Preview, the upstream code has been backported to the Red Hat Enterprise Linux client to provide pNFS block layout support.