MegaRAID 520-to-512 Byte Sector Format Tools

Problem Summary

I purchased two refurb'd Samsung PM1643a SSDs (OEM model: SLM5B-M3R8SS) and installed them in a Dell R740 Proxmox server with a PERC H330 controller. The drives were previously used in an enterprise storage array (likely Hitachi/HDS or EMC) and formatted with 520-byte sectors instead of the standard 512-byte sectors. This made them pretty unusable without flashing my H330 to IT-mode, which would require unacceptable downtime.

Drives Fixed

Slot	Serial	Result
4	B4YEK05H	✅ Fixed - /dev/sdf
5	B4YEK0GR	✅ Fixed - /dev/sdg

Symptoms

• Drive shows as "UGUnsp" (Unconfigured Good Unsupported) in perccli
• Logical Sector Size: 0 KB reported by controller
• Size: 0 KB - controller can't read capacity
• Drive is NOT exposed to Linux as /dev/sd* or /dev/sg*
• Samsung DC Toolkit cannot see the drive
• sg_format cannot access the drive
• All standard erase/format commands via perccli fail with "Operation not allowed"

Root Cause

smartctl -d megaraid,4 -i /dev/sda

Logical block size:   520 bytes
Physical block size:  4160 bytes

The PERC H330 (and most RAID controllers) only support 512-byte or 4096-byte sectors. The 520-byte format includes 8 extra bytes per sector for T10-DIF data integrity protection, used by enterprise storage arrays.

Solution

I discovered that while the PERC H330 won't expose the drive to Linux, smartctl can communicate with it via the MegaRAID passthrough IOCTL. So with the help of Claude Code I reverse-engineered the IOCTL interface from smartctl's source code and created custom tools to send SCSI commands directly to the drive.

Key Discovery

The MegaRAID driver (megaraid_sas) provides a passthrough interface at /dev/megaraid_sas_ioctl_node that allows sending SCSI commands to physical drives, even those marked as "unsupported."

Tools Created

1. mega_inquiry.c - Drive Identification Tool

Tests SCSI passthrough by sending an INQUIRY command to verify communication with the target drive.

Usage:

gcc -o mega_inquiry mega_inquiry.c
./mega_inquiry /dev/sda <target_id>

2. mega_format512.c - FORMAT UNIT Tool

Sends a FORMAT UNIT SCSI command to reformat the drive to 512-byte sectors.

Usage:

gcc -o mega_format512 mega_format512.c
./mega_format512 /dev/sda <target_id>

Note: This worked on Drive 1 (slot 4) - it reported "status 45" failure but actually succeeded!

3. mega_modesel.c - MODE SELECT + FORMAT Tool

Uses MODE SELECT to set block size to 512, then FORMAT UNIT to apply. This was needed for Drive 2 (slot 5) where the direct FORMAT UNIT approach didn't work.

Usage:

gcc -o mega_modesel mega_modesel.c
./mega_modesel /dev/sda <target_id>

4. check_size.c - Structure Validation Tool

Validates that the MegaRAID IOCTL structures match the expected sizes (404 bytes for megasas_iocpacket).

Step-by-Step Process

Step 1: Identify the Problem Drive

# List all drives - look for UGUnsp state and 0 KB size
/opt/MegaRAID/perccli/perccli64 /c0 /eall /sall show

# Confirm 520-byte sectors via smartctl passthrough
smartctl -d megaraid,<target_id> -i /dev/sda

Step 2: Try FORMAT UNIT First

# Compile and run
gcc -o mega_format512 mega_format512.c
./mega_format512 /dev/sda <target_id>

# Check if it worked (even if it reported failure!)
smartctl -d megaraid,<target_id> -i /dev/sda | grep "block size"

Step 3: If FORMAT UNIT Fails, Use MODE SELECT

# Compile and run
gcc -o mega_modesel mega_modesel.c
./mega_modesel /dev/sda <target_id>

# Check result
smartctl -d megaraid,<target_id> -i /dev/sda | grep "block size"

Step 4: Clear Controller Cache (IMPORTANT!)

The PERC controller caches the "unsupported" state. Even after successful format, perccli may still show UGUnsp. You MUST do one of:

Option A: Hot-reseat the drive (preferred)

• Physically unplug and replug the drive
• This forces the controller to re-discover it fresh

Option B: Reboot the server

• Only if hot-swap isn't possible

Step 5: Verify Success

# Check perccli - should show JBOD with correct size
/opt/MegaRAID/perccli/perccli64 /c0/e32/s<slot> show

# Check Linux sees it
lsscsi -g | grep -i samsu
lsblk

# Full verification
fdisk -l /dev/sdX
smartctl -a /dev/sdX

What Worked for Each Drive

Drive 1 (Slot 4, Serial B4YEK05H)

1. Ran mega_format512 - reported "status 45" failure
2. Checked smartctl - showed 512 bytes! (command actually worked)
3. SCSI rescan picked it up as /dev/sdf
4. No hot-reseat needed

Drive 2 (Slot 5, Serial B4YEK0GR)

1. Ran mega_format512 - reported "status 45" failure
2. Checked smartctl - still 520 bytes (didn't work)
3. Ran mega_modesel (MODE SELECT + FORMAT) - reported success
4. Checked smartctl - showed 512 bytes!
5. Controller still showed UGUnsp (stale cache)
6. Hot-reseated the drive (unplug/replug)
7. Controller recognized it as JBOD, appeared as /dev/sdg

Final Results

Property	Before	After
State	UGUnsp	JBOD
Size	0 KB	3.49 TiB
Logical Sector	520 bytes	512 bytes
Physical Sector	4160 bytes	4096 bytes
Linux Device	(none)	/dev/sdf, /dev/sdg
SMART Health	N/A	OK

How It Works

MegaRAID IOCTL Structure

struct megasas_iocpacket {
  u16 host_no;           // SCSI host number (from SCSI_IOCTL_GET_BUS_NUMBER)
  u16 __pad1;
  u32 sgl_off;           // Offset to scatter-gather list in frame
  u32 sge_count;         // Number of SG elements
  u32 sense_off;
  u32 sense_len;
  union {
    u8 raw[128];
    struct megasas_pthru_frame pthru;  // Passthrough frame
  } frame;
  struct iovec sgl[16];  // Scatter-gather list (userspace pointers)
};

Key SCSI Commands Used

• INQUIRY (0x12) - Identify drive
• MODE SENSE (0x1A) - Read current block descriptor
• MODE SELECT (0x15) - Set new block size
• FORMAT UNIT (0x04) - Apply new format
• READ CAPACITY (0x25) - Verify block size

Diagnostic Commands

# Check drive status in perccli
/opt/MegaRAID/perccli/perccli64 /c0/e32/s<slot> show all

# Read drive info via smartctl passthrough
smartctl -d megaraid,<target_id> -a /dev/sda

# List all drives
/opt/MegaRAID/perccli/perccli64 /c0 /eall /sall show
lsscsi -g

Prerequisites

On the Proxmox/Linux host:

apt-get install build-essential smartmontools sg3-utils lsscsi

Files in This Directory

File	Description
mega_format512.c	FORMAT UNIT tool - try this first
mega_modesel.c	MODE SELECT + FORMAT tool - use if format512 fails
mega_inquiry.c	INQUIRY test tool to verify passthrough works
check_size.c	Structure size validation tool
README.md	This documentation

Troubleshooting

FORMAT command reports failure (status 45) but might have worked

• Always verify with smartctl -d megaraid,X -i /dev/sda | grep block
• Status 45 (MFI_STAT_SCSI_IO_FAILED) doesn't always mean failure

smartctl shows 512 bytes but perccli still shows UGUnsp

• The controller has stale cache
• Hot-reseat the drive (physically unplug and replug)
• Or reboot the server

Neither FORMAT method works

• Try running FORMAT multiple times
• Some drives may need specific firmware
• May need to try in a different system with direct HBA access (LSI 9211-8i in IT mode)

Drive appears but can't be set to JBOD

• Usually means controller cache is stale
• Hot-reseat should fix this

References

• smartmontools source - megaraid.h
• smartmontools source - os_linux.cpp
• Level1Techs - Reformat 520 to 512 bytes
• setblocksize tool

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Created: January 2026 Successfully used on Samsung PM1643a (OEM: SLM5B-M3R8SS) on Dell R740 with PERC H330 Two drives reformatted from 520-byte to 512-byte sectors via MegaRAID passthrough IOCTL