Excluding directories from backup

Many applications generate large amounts of temporary or recoverable cache data (e.g. node_modules, thumbnail caches). This data usually does not need to be included in backups. The Cache Directory Tagging Specification standard exists to tag directories that contain such data. It defines a small tag file called CACHEDIR.TAG, which is stored at the root of a cache tree. These directories will then no longer be included in our backups.

For the automatically installed apps in mStudio, we have already placed the file in useful locations.

caution

Files that are excluded from the backup cannot be restored – not even by mittwald. Do not abuse the feature to save disk space by excluding important files from the backup.

TL;DR

Create a file CACHEDIR.TAG in each cache directory with exactly the following first line:

Signature: 8a477f597d28d172789f06886806bc55

Requirements

There are only a few requirements for a valid CACHEDIR.TAG file:

1. Filename: Must be exactly CACHEDIR.TAG (capitalisation important)
2. Signature header: The first line must read exactly: Signature: 8a477f597d28d172789f06886806bc55
3. Placement: A single CACHEDIR.TAG in the top-level cache directory is sufficient to mark the entire underlying directory tree as cache

When should you tag a cache directory?

Use CACHEDIR.TAG for directories whose content is regenerable - i.e. data that can be loaded from other sources or regenerated at any time. Typical examples:

• Package/build caches (Composer, npm, Yarn)
• Application caches (like TYPO3's var/cache directory)
• Generated artefacts (thumbnails)
• Backup directories managed by the application (e.g. Updraft)