Backup speed, battery and the network

Data Backup is built to stay out of your way. Automatic backups run at low priority so your own work comes first, step aside while the Mac is busy, and respect your battery and your data allowance — while a backup you start yourself with Back up now runs at full speed.

Gentle by default

• Automatic and scheduled backups run in the background at low priority, so the disk and processor go to your apps first. Back up now runs at full speed, because you're waiting for it.
• When your Mac is under heavy load, new automatic backups wait — "Waiting until your Mac is less busy." If the load lasts more than about 45 seconds while a backup is running, that backup is paused too: "Paused while your Mac is busy — … It picks up where it left off on its own." Nothing is lost; it carries on from where it stopped once things settle.

How much of your Mac background work may use is set in Preferences → Performance — see Performance modes and why Storage Studio works slowly on purpose.

%% caption: What a running backup does as conditions change
flowchart TD
  A[A backup is running] --> B{Is the Mac under heavy load for a while?}
  B -->|Yes| P[It pauses and resumes later where it stopped]
  B -->|No| C{Plugged in, or battery above 30 percent?}
  C -->|Yes| K[The Mac is kept from idle sleep until it finishes]
  C -->|No| S[The Mac may sleep and the backup carries on next time]

Battery and sleep

While a backup is running, Storage Studio keeps your Mac from falling asleep through inactivity, so the backup can finish — but only when it's safe for the battery:

• plugged in, or
• on battery above 30%, or
• a Mac without a battery.

If the battery drops to 30% or below while unplugged, Storage Studio lets the Mac sleep as normal; the backup simply carries on from where it stopped next time. Closing the lid of a laptop on battery puts it to sleep whatever any app does.

To keep automatic backups off battery altogether, tick The Mac is plugged in in the set's When to back up settings — see Backup schedules and triggers and Battery and power.

The network

• A speed check before automatic cloud backups. Before an automatic or scheduled backup to a cloud destination, Storage Studio uploads a small test file (4 MB) to that destination, times it and deletes it. If the upload runs at less than about 1 megabit a second, the backup waits: "Waiting for a faster connection — the upload to … is running at about …, which is too slow to finish a backup." The result is reused for up to six hours, so the test isn't repeated every time. If the speed can't be measured, the backup goes ahead. Back up now is never held back.
• Personal hotspots. Tick Not on a personal hotspot in a set's When to back up settings, and automatic cloud backups wait while macOS reports an expensive or low-data connection — "Waiting for Wi-Fi — you asked for backups to stay off hotspots."
• A dropped connection. An automatic backup that can't reach its destination waits quietly and tries again later instead of raising an alarm. Time when nothing was moving isn't counted in the speed and time-left shown on the card, which notes how much idle time it left out.

The first cloud backup

A first backup to the cloud uploads everything, in small encrypted pieces, so it can take a long time. Until the first cloud version has been checked, the card says: "Cloud backups upload in small encrypted pieces, so the first full backup is slow and can take a while. It keeps running in the background and resumes where it left off if interrupted — you can close this window." Later backups send only what changed and are much quicker.

Upload speed and Mac impact

When any set backs up to the cloud, an Upload speed and Mac impact section appears at the bottom of the Data Backup window. It shows how many uploads run at a time, any speed limit, and the performance mode it's matched to — or Paused while the Mac is busy.

Limit upload speed sets a ceiling from 1 to 100 MB/s, halved automatically while on battery. The speed limit applies to backups to Delta Trust and Delta Drive, and the uploads-at-a-time figure to Delta Trust; backups to your own cloud accounts aren't capped by these settings. See Delta Trust.

See also: Data Backup · What backup messages mean · Is Storage Studio slowing my Mac down?