How can I limit the amount of space taken up by Time Machine backup data?

Purpose

When backing up your Mac to your NAS using Time Machine, we suggest setting a quota to prevent the backup data from taking up too much space on your volume. You can do this via one or both of the methods provided in this article.

Resolution

Method 1: Assign a user quota

Do one of the following according to your DSM version:
- DSM 7.x: Go to Control Panel > User & Group > User and click Create.
- DSM 6.x: Go to Control Panel > User and click Create.
Enter a username and password. Continue through the wizard.
On the Assign shared folders permissions page, assign Read/Write permissions for your Time Machine folder.¹
On the Assign user quota page, enter a value for the Time Machine folder in the User Quota field and select a unit from the drop-down menu. Click Next.²³
On the Assign application permissions page, tick the Allow checkbox for AFP or SMB, depending on which file service you use. Then, click Next.
DSM 6.x users can skip this step.
Continue through the next prompts to complete the process.

Method 2: Assign a shared folder quota

Go to Control Panel > Shared Folder and select your Time Machine folder.¹⁴
Click Edit and go to the Advanced tab.
Tick Enable shared folder quota. Enter a value and select a unit from the drop-down menu.³
Click Save.

Notes:

You can check or set your Time Machine folder at Control Panel > File Services > Advanced > Bonjour > Set Time Machine Folders.
You can only assign user quotas for shared folders on models that support the Btrfs file system. If your NAS server only supports the ext4 file system, you can only assign user quotas for volumes.
The quota should be at least two to three times the size of your Mac's drive or the amount of data to be backed up. However, this is only a guideline and the space required also depends on how much data has changed since the last backup. If the remaining quota is less than the amount of changed data, the backup may fail.
You can only assign shared folder quotas on models that support the Btrfs file system.

Further reading

What should I do to fix the problem when connecting to Synology NAS using Time Machine?

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