linux filesystem resized automatically? without running resize2fs

Question

had to resize my EBS on AWS. i followed amazon's instructions which basically had me mount the EBS/HD on a second machine (as a secondary disk) and use 'gdisk' to resize the partition. Then they say to mount the EBS/HD back onto the original machine (restart it) and do 'resize2fs'.

However, after launching the original instance, 'df' showed the "available" and "use %" as being aware of the size I added using gdisk (on the other machine). I did not have to use resize2fs at all.

So, my question is: how did my system automatically resize the filesystem? As far as I'm aware, all i did was resize the partition, but not the filesystem? Amazon's documentation makes it seem like this is 2 different things. Do I still need to do the resize2fs? Will I run into complications later on if i don't? I'm afraid i'm running in some purgatory state even though everything seems fine (and i can save new files whereas previously the HD was 100% full before this).

The format is ext4 (GPT with protective MBR) if that helps.

Thanks!

Answer 1

It is a common feature for Linux images in EC2 to automatically call resize2fs on their root filesystem when they "wake up" with unused space available there. This is done by cloud-unit. There may be a log file of this action at /var/log/resizerrootfs.log or evidence of it elsewhere in /var/log.

It is because of this facility that you can initially launch an instance with a larger root volume than what was on the 8GB AMI, and are able to use all of the space without changing anything -- before the first boot, your new instance actually has an 8GB root filesystem, cloned byte-for-byte from the AMI snapshot.

The instructions are likely designed with safety and universality in mind. It's technically possible to modify the partition table and do the whole thing from a single running instance, but it's obviously a delicate operation if attempted.