What are block devices in Linux and how are they named?
Block devices are storage hardware (disks, SSDs, cards) exposed as files under /dev/, named by how they connect — sd* for SATA/USB, vd* for VMs, nvme* for NVMe.
A block device moves data in fixed-size chunks (blocks), as opposed to a character device (like a keyboard) that streams byte by byte. Linux represents every disk as a special file in /dev/, so the kernel can talk to wildly different hardware through one uniform interface — and so you can manage it with ordinary tools once a filesystem is mounted on it.
The name encodes the transport, which is why it tells you something about the hardware:
| Connection | Pattern | Example |
|---|---|---|
| SATA / SAS / USB | /dev/sdX |
/dev/sda, /dev/sdb |
| virtio-blk (paravirtualized VM disk) | /dev/vdX |
/dev/vda, /dev/vdb |
| NVMe SSD | /dev/nvmeXnY |
/dev/nvme0n1 |
| SD / MMC / eMMC card | /dev/mmcblkX |
/dev/mmcblk0 |
The trailing letter counts whole disks (sda, sdb, …); a trailing number is a partition on that disk (sda1, sda2). NVMe is the odd one out: nvme0n1 means controller 0, namespace 1, and its partitions get a p: nvme0n1p2.
Why it matters: a /dev/sdX letter is assigned in detection order, so it can change between boots if you add or remove a disk. That is exactly why filesystems are mounted by stable UUID rather than by device name (see the UUID card).
Go deeper:
Device file (Wikipedia) — the block vs character device distinction (buffered vs unbuffered) and the /devmodel.