Linux Sysadmin Notes

lsblk

4 min read

Tap Listen. On Android keep Chrome in the foreground and volume up.

lsblk

Concept

lsblk prints the block layer as a tree: physical disks, partitions, RAID, multipath, LVM, loop, and the filesystem sitting on top. It answers “what storage objects exist, how they stack, and where they are mounted?” without you assembling fdisk, pvs, and df by hand.

Device names (sda, nvme0n1, dm-2) are labels of convenience. They move after a bus rescan, a firmware change, or a cloud detach/attach. Identity is UUID, WWN, or serial.

Why it matters

  • Every destructive storage command (mkfs, wipefs, dd, lvremove, parted) needs a confirmed target
  • “Disk disappeared” vs “disk is there but the VG is inactive” vs “partition is there but unmounted” are different incidents; the tree tells you which
  • Cloud and SAN environments constantly remap names; lsblk -o NAME,SIZE,SERIAL,WWN,UUID,MOUNTPOINT is how you stay honest
  • Capacity tickets start here: is /data even on the 2 TB volume you just bought?

If you skip lsblk and trust /dev/sdb from last week, you will eventually format the wrong disk.

Mental Model

disk          TYPE=disk     (sda, nvme0n1, xvdf)
  partition   TYPE=part
    raid      TYPE=raid*
    mpath     TYPE=mpath    (/dev/mapper/mpathX)
    lvm       TYPE=lvm      (VG-LV)
      fs      FSTYPE + MOUNTPOINT

Read top-down to see what a disk became.
Read bottom-up (lsblk -s /dev/mapper/vg-data) to see what a volume sits on.

Empty MOUNTPOINT means “exists, not in the namespace you asked about” — which may still be mounted in another mount namespace (container).

Key Commands

# Default tree
lsblk
 
# The inspection line worth aliasing
lsblk -o NAME,SIZE,TYPE,FSTYPE,UUID,MOUNTPOINT,MODEL,SERIAL,WWN,TRAN
 
# Filesystem-oriented view
lsblk -f
 
# Inverse tree: physical disk under this LV
lsblk -s /dev/mapper/vg-data
 
# Disks only (no children)
lsblk -d -o NAME,SIZE,MODEL,SERIAL,TRAN,ROTA
 
# JSON for scripts
lsblk -J -o NAME,SIZE,TYPE,FSTYPE,UUID,MOUNTPOINT
 
# Bytes, no pretty units
lsblk -b -o NAME,SIZE,TYPE
 
# One device
lsblk -f /dev/nvme0n1
lsblk /dev/mapper/vg-root
 
# Include empty / kernel devices you usually hide
lsblk -a

TRAN is the transport (sata, nvme, iscsi, usb). ROTA=0 is SSD/NVMe. Pair with lsblk -t when you care about alignment and scheduler.

Common Failure Modes & Symptoms

SituationWhat lsblk showsNext step
“Where did the disk go?”Missing TYPE=diskCable/HBA/dmesg, multipath, cloud attach, SCSI rescan
About to mkfsConfirm SIZE, SERIAL, FSTYPE empty or expectedAbort if FSTYPE or MOUNTPOINT is populated
LV missingPV/part present, no lvm childpvs/vgs/lvs, vgchange -ay
Mounted on the wrong pathMOUNTPOINT not what fstab claimsmount and findmnt, UUID in fstab
Capacity “missing”Disk SIZE ≠ sum of childrenLeftover partition, unextended PV, thin pool metadata
Two names for one diskmpath plus underlying sd*Use the multipath node only
New cloud volume invisibleNo new disk after attachRescan; virtio vs NVMe device name change
Looks mounted, df disagreesDifferent mount namespacefindmnt -T on host and in container

Investigation Tips

  • Alias something like lsblk -o NAME,SIZE,TYPE,FSTYPE,UUID,MOUNTPOINT,SERIAL on every jump host.
  • After hot-add: echo "- - -" > /sys/class/scsi_host/hostN/scan for the right host, or the vendor equivalent. NVMe often just appears.
  • Never put /dev/sdX in fstab or automation. UUID or /dev/disk/by-id/… only.
  • On multipath, lsblk on the underlying path is how you accidentally wipe a live LUN. Operate on /dev/mapper/mpath*.
  • lsblk -s plus pvs -o+pv_used,pv_free is the fastest “why can I not extend this LV?” pair.
  • Loop devices (/dev/loop*) clutter the tree on systems that snap-pack packages. Filter with -e 7 if they drown the signal.

Personal Lessons Learned

  • The worst page I was on started with mkfs.xfs /dev/sdb because last week sdb was the new empty disk. After a rescan it was the database. lsblk -o NAME,SIZE,SERIAL,FSTYPE,MOUNTPOINT takes five seconds and is cheaper than a restore.
  • Cloud “I attached a 500 GB volume” tickets are usually “attached, not visible” or “visible, not mounted, not in fstab”. lsblk splits those in one look.
  • Inverse tree (-s) is how I finally stopped guessing which physical disk a thin LV lived on during a disk replacement.

Graph View

Backlinks