So I asked earlier about backing up the media files in my home lab, and thanks for the many who replied, I settled on sanoid / syncoid and has been running them for some time.

My setup:

pool1/data contains my media files
pool2/data backup

I use sanoid to make periodic snapshots of pool1/data, and then runs

syncoid pool1/data pool2/data

to replicate the snapshots to pool2.

This works fine, except I noticed that a lot of data is being copied even though pool1 did not change much. And now I just noticed pool2 ran out of space some time ago, while pool1 is currently only half full (pool1 & pool2 are the same size).

Is it because snapshots are somehow deduplicated when created on pool1, but is fully transferred to pool2 when syncoid runs? Is there something I can do to lower the usage on the backup pool? Thanks.