blockdev

Additional Notes

blockdev queries and changes low-level block device settings. It can report size, sector counts, read-only state, block size, and buffering behavior.

Use it carefully. Some options only query information, while others change device behavior.

Syntax

blockdev [options] device...

Parameters

• device: Block device such as /dev/sda, /dev/nvme0n1, or /dev/loop0.
• options: Query or set low-level block device properties.

Common Options

• --getsize64: Print device size in bytes.
• --getsz: Print size in 512-byte sectors.
• --getro: Show whether the device is read-only.
• --setro: Set device read-only.
• --setrw: Set device read-write.
• --getbsz: Get block size.
• --flushbufs: Flush buffers.
• --rereadpt: Ask the kernel to reread the partition table.

Examples

sudo blockdev --getsize64 /dev/sda

Show disk size in bytes.

sudo blockdev --getro /dev/sda

Check read-only state.

sudo blockdev --rereadpt /dev/sda

Reread the partition table.

Practical Notes

• Double-check device names with lsblk.
• Avoid changing block device state on mounted filesystems unless you know the impact.
• Many storage tasks are safer through higher-level tools such as lsblk, parted, fdisk, or blockdev query-only options.