The two performance-related settings are "Download strategy", which is a global setting, and "Multi-fetch size", which is an Account-level setting.
As with all performance tweaks, there are trade-offs.
The defaults, which suit most machines and plays nice with servers is:
- Download strategy: "delay writing metadata",
- Multi-fetch size: 1.
If you are using a resource-limited machine like a small virtual server or Raspberry Pi to run your backups, you can change "Download strategy".
If your email provider supports it, and you don't have tight memeory limits, increase "Multi-fetch size" for faster backups.
This is a global setting, affecting all account backups.
By default, imap-backup uses the "delay writing metadata" strategy.
As messages are being backed-up, the message text
is written to disk, while the related metadata is stored in memory.
While this uses a little more memory, it avoids rewiting a growing JSON file for every message, speeding things up and reducing disk wear.
The alternative strategy, called "write straight to disk", writes everything to disk as it is received. This method is slower, but has the advantage of using slightly less memory, which may be important on very resource-limited systems, like Raspberry Pis.
By default, during backup, each message is downloaded one-by-one.
Using this setting, you can download chunks of emails at a time, potentially speeding up the process.
Using multi-fetch will mean that the backup process will use more memory - equivalent to the size of the groups of messages that are downloaded.
This behaviour may also exceed the rate limits on your email provider, so it's best to check before cranking it up!