The amount of free space needed on the server is the sum of all file sizes for every profile on the client plus 1.5 times the larges file.
For example, if you are backing up 5 machines, each containing about 1GB of data, you need 5 GB of free space on the server, plus 1.5 times the largest file. If the file size of the largest file is 500 MB, you need at least 750 MB of additional free space on the server. Syncrify creates temporary files during the backup process, which requires this additional free space.
Similar free space is required on the client machine as well - meaning if the largest file on the client is 500 MB, you need additional 750 MB free on the client.
