Command referenceLinux / storage

Check the target
before you copy the command.

These are working examples for storage testing, disk health and legacy systems. Check the device names, software version and security assumptions in your own environment before using them.

01 / Storage benchmark

FIO — Flexible I/O Tester

FIO builds controlled I/O workloads for storage devices and files. The useful result is the one whose device, block size, queueing and cache behaviour match the question being asked.

Install

# Ubuntu / Debian
sudo apt update
sudo apt install fio

# RHEL / CentOS / Fedora
sudo dnf install fio

fio --version

Sequentially fill a device

Destructive operation

These commands permanently overwrite the target device. Verify the path, unmount every partition and confirm that no required data remains.

# 1 MiB sequential writes across the entire device
sudo fio \
  --name=filltest \
  --filename=/dev/nvme0n1 \
  --rw=write \
  --bs=1M \
  --direct=1 \
  --size=100% \
  --numjobs=1

Random-read baseline

sudo fio \
  --name=randread \
  --filename=/dev/nvme0n1 \
  --rw=randread \
  --bs=4k \
  --direct=1 \
  --numjobs=4 \
  --time_based \
  --runtime=60

Mixed workload

# 70% reads / 30% writes
sudo fio \
  --name=mixed \
  --filename=/dev/nvme0n1 \
  --rw=randrw \
  --rwmixread=70 \
  --bs=4k \
  --direct=1 \
  --numjobs=4 \
  --time_based \
  --runtime=60

--filename

The device or file under test.

--rw

The I/O pattern: read, write, randread or randwrite.

--bs

The block size used for each operation.

--direct=1

Bypasses the operating-system page cache.

--numjobs

The number of parallel workers.

--runtime

The sustained test duration in seconds.

02 / Disk health

Smartmontools — SMART monitoring

Smartmontools reads health, error and self-test information from supported storage devices. Treat SMART as evidence, not a guarantee: a healthy status does not replace redundancy or backups.

Install

# Ubuntu / Debian
sudo apt update
sudo apt install smartmontools

# RHEL / CentOS / Fedora
sudo dnf install smartmontools

smartctl --version

Inspect a device

# Device identity and SMART support
sudo smartctl -i /dev/sda

# Overall health assessment
sudo smartctl -H /dev/sda

# All SMART attributes
sudo smartctl -A /dev/sda

# Error log and prior self-tests
sudo smartctl -l error /dev/sda
sudo smartctl -l selftest /dev/sda

Run self-tests

# Short test
sudo smartctl -t short /dev/sda

# Extended test
sudo smartctl -t long /dev/sda

# Read the result after the stated completion time
sudo smartctl -l selftest /dev/sda

Enable the monitoring daemon

sudo systemctl enable --now smartd
sudo systemctl status smartd
sudo journalctl -u smartd

Operating practice

Schedule extended tests away from peak workloads, test alert delivery before relying on it, and investigate attribute trends rather than waiting for a binary failure flag.

03 / Compatibility

SSH — scoped legacy algorithms

Modern OpenSSH disables weak algorithms for good reason. Re-enable them only for equipment that cannot be upgraded, only on trusted paths, and only for the specific host that needs them.

Typical negotiation errors

Unable to negotiate with 10.10.201.2 port 22:
no matching key exchange method found.
Their offer: diffie-hellman-group14-sha1

Unable to negotiate with 10.10.201.2 port 22:
no matching host key type found.
Their offer: ssh-rsa

One-time connection

ssh \
  -oKexAlgorithms=+diffie-hellman-group14-sha1 \
  -oHostKeyAlgorithms=+ssh-rsa \
  user@10.10.201.2

Host-specific configuration

# ~/.ssh/config
Host legacy-appliance
    HostName 10.10.201.2
    User user
    KexAlgorithms +diffie-hellman-group14-sha1
    HostKeyAlgorithms +ssh-rsa
    PubkeyAcceptedAlgorithms +ssh-rsa

mkdir -p ~/.ssh
chmod 700 ~/.ssh
chmod 600 ~/.ssh/config
ssh legacy-appliance

Security boundary

SHA-1-based RSA and old Diffie-Hellman groups have known weaknesses. Never enable them globally. Record the exception, isolate the device and remove the override when the equipment is replaced.