Topics Covered

On this page

1. NOT EXECUTED
2. Launch, attach, format, mount
3. The three that go wrong
4. An EBS volume is in ONE availability zone
5. Volume types, and how to choose
6. Snapshots

NOT EXECUTED

This repository has no cloud account, and none will be created. Creating one requires a payment card and accepts a billing relationship, which is not something a study repository should do on anyone's behalf.

So this file is what you type, with the traps marked. It has never been run here, and nothing in the notes claims an output for it.

The runnable half is 04_storage.py, which compares block, file and object semantics and cost.

---

Launch, attach, format, mount

aws ec2 run-instances --image-id ami-xxxx --instance-type t3.micro \
  --key-name mykey --security-group-ids sg-xxxx --subnet-id subnet-xxxx

aws ec2 create-volume --size 20 --volume-type gp3 \
  --availability-zone us-east-1a
aws ec2 attach-volume --volume-id vol-xxxx --instance-id i-xxxx \
  --device /dev/sdf

Then, on the instance:

lsblk                                   # find it -- it appears as /dev/nvme1n1
sudo file -s /dev/nvme1n1               # "data" means NO FILESYSTEM yet
sudo mkfs -t xfs /dev/nvme1n1           # DESTROYS anything on it
sudo mkdir /data && sudo mount /dev/nvme1n1 /data
sudo blkid                              # get the UUID
echo 'UUID=<uuid> /data xfs defaults,nofail 0 2' | sudo tee -a /etc/fstab

The three that go wrong

1. mkfs on the wrong device. lsblk first, every time. Formatting the root volume ends the instance.

2. Forgetting /etc/fstab. The volume is not mounted after a reboot and your application starts writing to the root disk instead — silently, until it fills.

3. nofail omitted. If the volume is missing at boot, the instance hangs in the boot sequence and you cannot SSH in to fix it. nofail turns a catastrophe into a missing directory.

An EBS volume is in ONE availability zone

You cannot attach us-east-1a's volume to an instance in us-east-1b. To move it: snapshot it (snapshots go to S3, which is regional), then create a volume from the snapshot in the other AZ.

That constraint is why "an EBS volume attaches to one instance" is really "one instance, in one AZ" — and it is why EFS exists.

Volume types, and how to choose

Type	For	Notes
gp3	almost everything	IOPS and throughput are independent of size
gp2	legacy	IOPS scale with size — the reason gp3 replaced it
io2 Block Express	a demanding database	expensive, and supports Multi-Attach
st1	big sequential reads	HDD; terrible at random I/O
sc1	cold archive on a disk	cheapest, slowest

gp3 is the default answer. Under gp2 you would over-provision a volume purely to buy IOPS; gp3 unbundled them.

Snapshots

aws ec2 create-snapshot --volume-id vol-xxxx --description "before upgrade"

Snapshots are incremental — only changed blocks — and stored in S3. Deleting an intermediate snapshot does not break later ones; AWS re-parents the blocks. But snapshots of a mounted, busy filesystem may be crash-consistent rather than clean: freeze or unmount for a database.