[Arnavion]

I did create the block files as sparse originally (using `truncate`), but at some point in the process they became realized on disk. Don't know if it was the losetup or the mdadm or the cryptsetup. I didn't really worry about it, since the block files need to be synced to the WebDAV server in full anyway.

[hardwaresofton]

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?

[Arnavion]

The stack is XFS inside cryptsetup inside mdraid on top of losetup. The directory containing the losetup block files could be `rclone mount`'d from the WebDAV server, but that would make the setup unavailable if I didn't have network access. So instead I chose to have the block files in a regular directory, and I make sure to `rclone sync` that directory to the WebDAV server when I make changes in the XFS layer. Manually syncing also lets me run `sync` and watch the `rclone sync` output, which gives me greater confidence that all the layers have synced successfully.

>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.

Right. Let's say I update two files in the XFS layer. Those writes eventually result in three blocks in the lowest layer being modified. So now the `rclone sync` will need to do a `PUT` request to replace those three blocks on the WebDAV server, which means it'll upload 3MB of data to the server.

[hardwaresofton]

Thanks for the explanation, this makes perfect sense now, didn't realize the syncing was manual/separate.