Invalid command operation code WRITE SAME

The error “Invalid Command Operation Code: WRITE SAME” typically appears in environments where a command sent to a storage device (usually a hard drive or SSD) is not recognized or supported. This is most commonly seen in Linux-based systems, especially when performing low-level disk operations or using tools like blkdiscard, dd, fstrim, or sg_unmap.

Understanding this error requires familiarity with SCSI (Small Computer System Interface) command sets. WRITE SAME is a SCSI command used to write the same data block to multiple locations, often employed to optimize performance during data clearing or trimming. However, not all drives support this command, leading to compatibility issues.

What Triggers the “WRITE SAME” Error?

You may encounter this error in situations like:

Running blkdiscard on a block device.

Formatting a disk using a tool that attempts to zero data blocks quickly.

Attempting a TRIM/discard operation on SSDs.

Using virtualized disks that do not fully emulate SCSI behavior.

The full error might look something like this in dmesg or a terminal output:

blk_update_request: I/O error, dev sdb, sector 0

Buffer I/O error on dev sdb, logical block 0

WRITE SAME failed. Manually zeroing.

sg_write_same failed: Invalid command operation code

This means the tool attempted to use the WRITE SAME command, but the storage device responded with an error indicating it doesn't support that operation code.

Technical Background: What Is WRITE SAME?

WRITE SAME (0x93) is a command defined in the SCSI standard. It is used to write a single block of data across multiple locations on the disk, often for:

Efficiently zeroing large areas of a disk.

Secure erase procedures.

Optimization of disk I/O during discard operations.

WRITE SAME is intended to reduce the overhead compared to writing block-by-block manually. However, many drives—especially older ones, some SSDs, and USB flash drives—do not implement this command, leading to compatibility problems.

How to Fix or Work Around the Error

1. Use Manual Zeroing Instead

If WRITE SAME fails, you can manually zero the drive using dd:

sudo dd if=/dev/zero of=/dev/sdX bs=1M status=progress

Replace /dev/sdX with your actual device. This method is slower but universal.

2. Disable Discard/TRIM Features

If the error occurs during formatting or filesystem creation (e.g., ext4 or XFS), try disabling discard:

sudo mkfs.ext4 -E nodiscard /dev/sdX

This prevents the tool from sending WRITE SAME or TRIM commands during setup.

3. Update Storage Device Firmware

Some storage devices can be updated with firmware that may add or fix support for WRITE SAME or related commands. Check the manufacturer’s website.

4. Use a Different Tool or Flag

Tools like blkdiscard or fstrim can fail on unsupported devices. Try using --no-wsame or alternatives (if available), or skip discard operations entirely if the device doesn’t benefit from them.

Example:

sudo blkdiscard --no-wsame /dev/sdX

Note: Not all versions of blkdiscard support this flag.

5. Virtual Environments: Check Disk Emulation

In virtual machines (e.g., VMware, VirtualBox, KVM), the virtual disk controller might not support WRITE SAME. Switching disk types (e.g., from SCSI to VirtIO) or adjusting controller settings may help.

When to Ignore It

If your disk still works fine and the operation falls back to a compatible method (like manual zeroing), you might not need to do anything. Some Linux utilities handle the fallback gracefully and continue operation despite the error.

Conclusion

The “Invalid Command Operation Code: WRITE SAME” error stems from the mismatch between software attempting a modern, efficient SCSI command and hardware that lacks support for it. While it’s not usually a fatal issue, it can interrupt disk formatting, zeroing, or discarding operations. The solution often lies in using more universally compatible tools or bypassing unsupported commands entirely.

Let me know if you want a script to detect WRITE SAME support on your drives or logs interpreted from a specific error message.