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Singularity's Persistent Overlays

This is a requirement for applications that must have write privilleges, such as Oracle Database images.

If you want to use a SIF container as though it were writable, you can create a directory, an ext3 file system image, or embed an ext3 file system image in SIF to use as a persistent overlay.

Usage

we can use persistent overlay in the following scenarios.

File system image overlay

You can use tools like dd and mkfs.ext3 to create and format an empty ext3 file system image, which holds all changes made in your container within a single file.

Features

  • Easy to transport (it is a single file)
  • Faster than a directory (for HPC tasks)
  • No privillege required
  • Empty space information required

Directory Overlay

Easier to use, but can't be transported as easily, also must be root.

Overlay embedded in SIF

It is possible to embed an overlay image in the SIF file that holds a container. This allows the read-only container image and your modifications to it to be managed as a single file.

Final note

If you mount your container without the --overlay directory, your changes will be gone.