Filesystems

In Linux and Unix-like systems, /dev/sdf refers to a device file which represents a particular storage device in the system. These device files are located in the /dev directory, which contains special files that represent devices that are either part of the system's hardware or are virtual devices provided by the software.

Breakdown of /dev/sdf:

• /dev/ is the directory in Unix and Unix-like operating systems where device files are stored.

• sdf usually represents a SCSI disk device. Here, the sd prefix stands for "SCSI disk," a naming convention used for SCSI or SATA drives and some other storage devices, even if they are not physically SCSI devices.

• f in sdf indicates the order of the device. Devices are typically named in order such as sda, sdb, sdc, and so on, based on the order they are discovered or configured in the system. The letter f means it is the sixth device because a would be the first, b the second, and so forth.

Can it be any character?

• The final character in device names like /dev/sdf can vary and generally depends on the number of storage devices attached. It follows alphabetical order after a, which stands for the first detected device. If you have more than 26 devices, the naming will start to include additional letters, like sdaa, sdab, etc.

It's important to note that while you can specify /dev/sdf in your command, the actual device name within the operating system can differ due to how the modern Linux distributions handle device naming (for instance, using /dev/xvd or /dev/nvme prefixes instead). 

You should check how your specific instance's operating system sees attached volumes, often using commands like lsblk or fdisk -l.

The difference between /dev/xvdf and /dev/sdf in the context of Linux environments pertains mainly to the naming conventions and interfaces used by different types of virtualized storage devices.

Historical and Technological Background:

• /dev/sdf: This is the traditional naming scheme for disk devices in Linux, typically used for SCSI, SATA, or similar hardware interfaces. In a physical server, these devices would be physically connected SCSI or SATA drives. However, in a virtualized environment like Amazon EC2, /dev/sdf might be used as a logical name in CLI commands to attach a volume, even though the underlying technology is not directly related to traditional SCSI interfaces.

• /dev/xvdf: This naming convention comes from the Xen virtualization technology, which Amazon EC2 used extensively in the past (and may still use under certain configurations). Devices named with an xvd prefix represent virtual disks in a Xen-based virtualized environment. The xvd prefix is used instead of sd to avoid naming conflicts and to help script and system administrators distinguish between physical disks and virtual disks.

Usage in EC2:

In the AWS EC2 context, when you specify a device as /dev/sdf during the attachment of an EBS volume, AWS internally maps it to a Xen virtual disk device like /dev/xvdf within the EC2 instance, depending on the instance type and the underlying virtualization type. This mapping is handled by AWS and is transparent to the user. For example, while you may attach a volume as /dev/sdf, the operating system inside the EC2 instance might see it as /dev/xvdf.

Modern Instances and NVMe:

• NVMe Devices: Newer EC2 instance types use NVMe-based EBS volumes, which follow a completely different naming scheme, such as /dev/nvme1n1. These devices offer better performance compared to xvd devices due to the efficiency of the NVMe protocol over traditional SCSI commands used by xvd devices.

Practical Implications:

• Compatibility and Scripts: For backward compatibility, AWS allows users to specify traditional device names (/dev/sdf, etc.) when attaching volumes. Scripts and software that were originally designed for older Linux systems can still function without modification in newer, virtualized environments.

• OS Visibility: Despite specifying /dev/sdf in AWS commands, you should check how your specific EC2 instance's operating system recognizes the device. Modern distributions and certain configurations might display devices as /dev/xvdf, /dev/nvme1n1, etc., depending on the instance type and virtualization technology.

In summary, /dev/sdf and /dev/xvdf represent different layers of abstraction over physical and virtual storage devices in a Linux environment, with their usage nuanced by the context of virtualization technologies like Xen and the specific configurations of cloud platforms like AWS EC2.