Default partition type for Instance Store disks?

Question

I have created an EC2 instance with an instance store automatically mounted to it. Although I can use the mounted disk correctly I can fdisk command complains that the disk does not have a valid partition table (here is the output):

Disk /dev/xvdb: 4289 MB, 4289200128 bytes
255 heads, 63 sectors/track, 521 cylinders, total 8377344 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x00000000

Disk /dev/xvdb doesn't contain a valid partition table

I was curious what cause this and if there is really a partition there what is the type of it?

Answer 1

There is no partition type, because there is no partition table.

Linux supports the use of an entire volume for a single filesystem without partitioning the disk, and that's how ephemeral disks are typically formatted by default -- when they are formatted at all -- as a single ext2/ext3 filesystem spanning the entire volume.

Many instance store volumes are pre-formatted with the ext3 file system. SSD-based instance store volumes that support TRIM instruction are not pre-formatted with any file system. However, you can format volumes with the file system of your choice after you launch your instance.

http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/add-instance-store-volumes.html

If you want a different filesystem, you can replace the default (if provided) by using the entire volume for the new filesystem, e.g. mkfs -t ext4 /dev/xvdX, or, you can start from scratch and force the creation of a partition table.

Remember that ephemeral volumes are ephemeral... they will survive an instance reboot but not an instance stop or termination.