---
title: Library syncing vs rebuilds
url: https://photostructure.com/guide/sync-vs-rebuild/
description: Library syncing vs rebuilding
date: 2020-10-18
keywords: library, sync, library-rebuild, volumes, deduplication, metadata
---


PhotoStructure has several strategies to ensure your [library](/faq/library/)
matches your filesystem.

## 🩳 Briefly

A "sync" focuses on _your filesystem_. Sync updates your library with changes
made to your filesystem.

A "rebuild" focuses on what's _already in your library_. Rebuild updates your
database to reflect improved PhotoStructure algorithms.

## 🔍 Library syncs

PhotoStructure runs "sync" jobs for the directories in your [scan
paths](/guide/scan-paths/), and if you've enabled automatic
scanning, all your mounted [volumes](/guide/what-is-a-volume/).

Synchronization has 3 steps:

1. The directory is recursively scanned for files that might be eligible for
   being imported into your library ([see this post for
   details](/faq/why-is-my-file-missing/)). The progress bar says "Scanning..."
   during this step.

2. Those eligible files are individually examined. Only those files (or their
   sidecars) that have changed since the last sync will be re-imported. If
   you've opted for [automatic
   organization](/getting-started/automatic-library-organization/), new files
   and sidecars will be copied into your library. The progress bar says
   "Processing..." during this step.

3. The prior contents of the directory, according to your PhotoStructure's
   library database, is then scanned, and missing files are handled
   appropriately.

PhotoStructure is designed to handle very large libraries. **Don't worry about
stopping PhotoStructure in the middle of a sync!** PhotoStructure will pick up
where it left off as soon as you restart.

PhotoStructure runs a sync for all volumes daily, but you can manually start a
resync via the navigation menu.

PhotoStructure also monitors your volumes, and as new volumes are mounted, it
will sync them automatically.

## 🔬 Library rebuilds

Your library may be automatically [rebuilt](/faq/library-rebuilds/) when you
upgrade to a new version of PhotoStructure that improves heuristics for
de-duplication, improves metadata inference, or adds new tagging curators.

Rebuilds are expensive! They may require:

- re-extraction of metadata
- recalculation of image hashes
- regeneration of previews
- re-transcoding of any missing videos
- re-aggregation of assets

Rebuilds should not be necessary except after version upgrades.

## See also

[Why is PhotoStructure rebuilding my library?](/faq/library-rebuilds/)