> ## Documentation Index
> Fetch the complete documentation index at: https://developer.upsun.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Backup consistency: why it matters and how Upsun handles it

> Learn what backup consistency means at the filesystem and service levels, and how Upsun balances consistent backups with zero-disruption live backups.

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<PostMeta data={{ author: ["ralt"], date: "2026-07-21T13:00:00.000Z" }} />

A backup is only useful if you can restore it. Copying the bytes is the straightforward part. The hard part is making sure those bytes still make sense together once you restore them. That property is called consistency, and it lives at 2 levels: within a single filesystem, and across the services that make up your application.

## Consistency at the filesystem level

Picture what happens when you snapshot a disk while the application is writing to it. A file's data blocks may already be on disk while the filesystem metadata (the structures that describe what lives where, like the superblock) still says something else. Restore that snapshot and you get a filesystem that disagrees with itself. Repair tools can sometimes fix it, and sometimes they can't.

Linux has a utility for this called [fsfreeze](https://man7.org/linux/man-pages/man8/fsfreeze.8.html). It flushes all in-flight IO to disk and blocks any new writes until the filesystem is thawed. While it's frozen, you can copy the filesystem and know the copy is valid: it will mount, and it will restore, every time.

On Upsun, consistent backups freeze every filesystem this way before the snapshot is taken. You never have to think about it.

## Consistency between services

Freezing 1 filesystem stops being enough the moment your application is more than a static website. Add a database next to your app and a second consistency problem appears, at a higher level this time.

Take the most common example: file uploads. A user uploads an image, your application writes the file to disk, then stores its path in the database. That's 2 writes, on 2 services, a few milliseconds apart.

Now a backup runs in between. The database snapshot has the row with the path. The filesystem snapshot doesn't have the file yet. Each snapshot is valid on its own, and together they describe an application state that never existed. Restore them and your database points at files that aren't there.

## How a consistent backup works on Upsun

When you click the backup button in the Console (or run `upsun backup:create`), here is what happens by default:

1. Incoming requests are held at the edge. They're not dropped, they wait.
2. Every service pauses its own work. The database, for example, locks its tables so no transaction is left mid-flight.
3. Once all services report they're paused, every filesystem is frozen.
4. Snapshots are taken across all containers, capturing the same moment in time.
5. Everything thaws, unlocks, and the held requests are released.

The result is a backup where all services agree with each other. The cost is a short window, usually 15 to 30 seconds, where your site is paused. No requests are lost, but visitors during that window are waiting on a site that feels frozen. The details are in the [backups and downtime documentation](/docs/environments/backup#backups-and-downtime).

## Live backups: zero disruption, weaker guarantees

That pause is why live backups exist. A live backup skips the coordination entirely: no held requests, no locked tables, no frozen filesystems. Snapshots are taken on the fly while the application keeps serving traffic. The impact on your live site is zero.

The trade-off is weaker guarantees. At the filesystem level you're still fine in practice: the snapshot is atomic at the block level, restoring it is like booting a machine after a power loss, and the filesystem journal replays whatever was in flight. Between services though, the guarantee goes away. There's a chance (not a certainty, a chance) that a live backup captures a state like the upload example above, and restores with some data out of sync.

[Automated backups](/docs/environments/backup#backup-policy) on Upsun are always live. The default policy takes 1 backup per day on production, and you can raise that to several per day. Take 4 live backups a day and one of them might occasionally catch your application mid-write. We think that's the right default: a scheduled backup shouldn't pause your production site 4 times a day.

Engineering is trade-offs, and this one is explicit. The default optimizes for zero disruption. When you want the stronger guarantee, [manual backups](/docs/environments/backup#create-a-manual-backup) are consistent by default. You can also take a manual live backup with the `--live` flag, though it's hard to find a reason to prefer it over the scheduled ones.

```bash theme={null}
upsun backup:create --live
```

## Which one should you use?

Let the scheduled live backups run as your safety net. They restore fine most of the time, and they cost you nothing in availability.

Before anything risky, like a schema migration, a big import, or a deploy you're unsure about, take a manual backup and accept the short pause. That's the backup you'll actually want to restore from, and you want it to be the consistent kind. If your site is busy, run it during off-peak hours.

If you're curious how the snapshots themselves work under the hood (incremental, deduplicated, built on Ceph), we wrote about that in [High-performance backups: How Ceph snapshots enable incremental full backups](/posts/how-it-works/how-do-we-do-incremental-but-really-full-backups-on-top-of-ceph).