Ahh OK, I think I see -- since the block files are synced in full, you are always swapping blocks and doing ~1MB of writing no matter what.

> I use a WebDAV server for storing backups (Fastmail Files). The server allows 10GB usage, but max file size is 250MB, *and in any case WebDAV does not support partial writes*. So writing a file requires reuploading it, which is the same situation as S3.

This is the part I absolutely missed. I was wondering how you were ensuring 1MB writes -- whether it was at the XFS level or mdraid level...

I think another thing that is missing which I'm inferring (hopefully correctly) is that you've mounted your webdav server to disk. So your stack is:

- LUKS

- mdraid

- losetup

- webdav fs mount

Is that correct?