When running
rsync -Paz /home/sbird "/run/media/sbird/My Passport/sbird"
As told by someone, I run into a ran out of storage error midway. Why is this? My disk usage is about 385 GiB for my home folder, and there is at least 800 GiB of space in the external SSD (which already has stuff like photos and documents). Does rsync make doubly copies of it or something? That would be kind of silly. Or is it some other issue?
Note that the SSD is from a reputable brand (Western Digital) so it is unlikely that it is reporting a fake amount of storage.
Could it be you have lots of tiny files and/or a rather large-ish block size on your SSD?
You can check the block size with
sudo blockdev --getbsz /dev/$THE_DEVICE.using the command returns 512 for the external SSD and 4096 for the SSD in my laptop. What does that mean?
Imagine your hard drive like a giant cupboard of drawers. Each drawer can only have one label, so you must only ever store one “thing” in one drawer, otherwise you wouldn’t be able to label the thing accurately and end up not knowing what went where.
If you have giant drawers (a large block size), but only tiny things (small files) to store, you end up wasting a lot of space in the drawer. It could fit a desktop computer, but you’re only putting in a phone. This problem is called “internal fragmentation” and causes files to take up way more space than it would seem they need.
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However, in your case, the target block size is actually smaller, so this is not the issue you’re facing.