{"id":44323,"date":"2024-08-02T16:44:25","date_gmt":"2024-08-02T20:44:25","guid":{"rendered":"https:\/\/mjtsai.com\/blog\/?p=44323"},"modified":"2024-08-05T21:42:41","modified_gmt":"2024-08-06T01:42:41","slug":"snapshots-arent-backups","status":"publish","type":"post","link":"https:\/\/mjtsai.com\/blog\/2024\/08\/02\/snapshots-arent-backups\/","title":{"rendered":"Snapshots Aren’t Backups"},"content":{"rendered":"

Howard Oakley<\/a>:<\/p>\n

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What is different is that restoring a whole volume from a snapshot is a one-way trip, and there is no undo. This is because snapshots subsequent to that used to restore from will be removed, and you won’t then be able to ‘roll forward’ to a later snapshot. That contrasts with a normal backup, where items remain available from any other backup that is retained in the backup store.<\/p>\n

[…]<\/p>\n

Because snapshots share the same container as the current volume, and share many file extents with them, they are prone to common errors. In particular, common file extents make it more likely that faults occurring in extents and data storage will affect them both. This is particularly important as one of the most common file system errors that corrupts data in files occurs when extents for two separate files overlap. A snapshot is thus more vulnerable than a backup on a different disk, or even one in a different container on the same physical store.<\/p>\n

[…]<\/p>\n

Snapshots do have one specific advantage over backups when it comes to their coverage. As they include the whole file system metadata for the volume, no items present in that volume are excluded from its snapshots. If you want to restore an item that has been excluded from backups made of any volume, you can therefore do that from its latest snapshot, if that item was present in the volume at the time that was made.<\/p>\n

The only disadvantage to this is that snapshots can be disproportionately large compared to volume backups.<\/p>\n<\/blockquote>\n

Snapshots are a great tool, but they don’t replace backups. The combination can be powerful. All my clone backups are now to APFS drives that make a new snapshot for each backup. I would like to be able to restore previous versions of files or folders from a year ago or more. Every once in a while I archive a clone drive and stop updating it. But I don’t have enough drives in rotation to keep a version for each week or month. Snapshots make that possible, albeit with less redundancy. Unfortunately, Mac backup software has kind of regressed in that it no longer provides great tools for browsing and searching old versions, but at least with snapshots we can easily and efficiently store them.<\/p>\n\n

Previously:<\/p>\n