Block devices, naming, and disk identification · Linux

Every disk on a Linux system has a name. The naming scheme depends on the bus the disk is attached to, the kernel driver, and how the disk is partitioned. Knowing the naming scheme is the first step in any storage diagnostic.

The major naming schemes

Read-only / Safelsblk disks— lsblk -d shows whole disks only (no partitions). The TRAN column shows the transport: sata for sd*, nvme for nvme*, virtio for vd*, and loop for loop* devices. Knowing the bus type is the first step in identifying the disk.

$ lsblk -d -o name,size,type,tran

NAME    SIZE TYPE TRAN
sda    100G disk
sdb    500G disk
nvme0n1 1.0T disk
vda     20G disk
loop0    1G loop

Pattern	Bus	Notes
`sda`, `sdb`, …	SCSI/SATA/USB	Most common on physical hardware. Disk N is `sdN`; partition M is `sdNM`.
`nvme0n1`, `nvme1n1`, …	NVMe	Namespace-based naming. `nvme0n1` is the first namespace of the first NVMe controller.
`vda`, `vdb`, …	virtio-blk	Virtual disks in KVM/QEMU guests.
`xvda`, `xvdb`, …	Xen virtio	Older Xen-style virtual disks.
`loop0`, `loop1`, …	Loop device	Used for mount images, snap packages, container overlays.
`mmcblk0`, …	MMC/SD	eMMC, SD cards.

The partition naming

Each whole-disk device has partition children. The naming convention is <disk><partition>:

Read-only / Safelsblk tree— lsblk shows the parent-child relationship: a whole disk has partitions, and partitions may have logical volumes on top. Mount points show where each filesystem is attached.

$ lsblk -o name,size,type,fstype,mountpoint

NAME          SIZE TYPE FSTYPE MOUNTPOINT
sda          100G disk
sda1          1G part ext4 /boot
sda2         99G part
├─sda2      5G lvm  ext4 /
└─sda2     94G lvm  ext4 /var
sdb         500G disk
sdb1        500G part ext4 /data
nvme0n1   1.0T disk
nvme0n1p1  1.0T part ext4 /opt
vda          20G disk
vda1         20G part ext4 /

Stable identifiers

The stable identifiers that survive reboots and hardware changes:

Read-only / Safeblkid output— blkid prints the UUID, TYPE, and LABEL for every block device. UUIDs are stable across reboots and hardware changes. The LABEL is human-readable but can be duplicated if the operator is careless. Use UUID by default; use LABEL only when readability matters.

$ blkid

/dev/sda1: UUID="1a2b3c4d-..." TYPE="ext4" PARTUUID="..."
/dev/sda2: UUID="5e6f7a8b-..." TYPE="LVM2_member" PARTUUID="..."
/dev/mapper/vg0-root: UUID="9a8b7c6d-..." TYPE="ext4"
/dev/sdb1: UUID="f1e2d3c4-..." TYPE="ext4" LABEL="data"
/dev/nvme0n1p1: UUID="a1b2c3d4-..." TYPE="ext4" PARTUUID="..."
/dev/vda1: UUID="p1p2p3p4-..." TYPE="ext4"

Finding which disk holds a mount point

When a filesystem fills up, the first question is “which disk?”. Three answers, in increasing detail:

Read-only / Safedf— df shows the logical volume holding the mount. The path /dev/mapper/vg0-var is a symlink to /dev/dm-X. To find the physical disk, follow the LVM chain.

$ df /var/log

Filesystem      Size  Used Avail Use% Mounted on
/dev/mapper/vg0-var 94G  82G  12G  87% /var

Read-only / Safelvs— lvs shows the LV to PV mapping. var uses 94 GB on /dev/sda2. To find the physical disk, follow /dev/sda2 to its bus path.

$ lvs -o+devices vg0

  LV   VG   Attr    LSize   Devices
root vg0  -wi-ao  5.00G  /dev/sda2(0)
var  vg0  -wi-ao 94.00G  /dev/sda2(2048)

Read-only / Safeby-path— /dev/disk/by-path contains stable hardware paths. pci-0000:01:00.0-nvme-1 is the NVMe slot on the first PCIe bridge. This is the most stable identifier for physical hardware.

$ ls -l /dev/disk/by-path/

pci-0000:00:1f.2-ata-1.0 -> ../../sda
pci-0000:00:1f.2-ata-2.0 -> ../../sdb
pci-0000:01:00.0-nvme-1 -> ../../nvme0n1