How to Flash NRU-51 with A/B Partition Layout for Jetpack6.0

This guide walks you through the process of flashing an NRU-51V using an A/B rootfs partition layout on an external 128GB NVMe SSD.

1. Prerequisites

Before starting the flashing process, make sure you have:

• A host PC with Ubuntu 20.04 or 22.04

• Jetpack 6.0 L4T BSP environment properly set up

• Jetson board in RCM (Recovery) mode

• A working USB cable (preferably direct to motherboard, not via HUB)

• External NVMe SSD connected to the target device (Take 128GB SSD for example.)

• Your custom partition layout file:

  Linux_for_Tegra/tools/kernel_flash/flash_l4t_nvme_rootfs_ab.xml

✅ Tip: You can verify the Jetson is in recovery mode with:

lsusb

Look for:

0955:7035 NVidia Corp. APX

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2. Check Disk Size and Sector Count

Before flashing, confirm the real SSD capacity in jetson and calculate num_sectors used in the XML file.

sudo blockdev --getsz /dev/nvme0n1

Example output for a 128 GB NVMe SSD:

250069680

• 1 sector = 512 bytes

• 250069680 × 512 = 128,035,676,160 bytes ≈ 119.24 GiB ≈ 128 GB

This number must match the num_sectors value in your XML file.

Example Partition Layout (128 GB)

Partition	Size (Bytes)	Size (GB)	Purpose
A_kernel + DTB	134,217,728	0.13 GB	Slot A kernel
B_kernel + DTB	134,217,728	0.13 GB	Slot B kernel
ESP + ESP_alt	2 × 67,108,864	0.13 GB	EFI partitions
UDA	419,430,400	0.4 GB	Reserved by NV for OTA process. and, the UDA partition size to be a multiple of 512 Bytes
Reserved	502,792,192	0.5 GB	Reserve space in case there is any partition change required in the future
APP (A)	48,318,382,080	48 GB	Rootfs slot A
APP_b (B)	48,318,382,080	48 GB	Rootfs slot B
GPT + misc	Remainder		Partition table + margin

✅ Sample device tag in XML

<device type="external" instance="0" sector_size="512" num_sectors="250069680">
....
<!-- RootFS A -->
<partition name="APP" id="1" type="data">
    <allocation_policy> sequential </allocation_policy>
    <filesystem_type> basic </filesystem_type>
    <size> 48318382080 </size>
    <!-- 48 GB = 45 GiB -->
    <!-- 45*1024*1024*1024=48318382080 -->
    <file_system_attribute> 0 </file_system_attribute>
    <allocation_attribute> 0x8 </allocation_attribute>
    <align_boundary> 16384 </align_boundary>
    <percent_reserved> 0 </percent_reserved>
    <unique_guid> APPUUID </unique_guid>
    <filename> APPFILE </filename>          
</partition>

<!-- RootFS B -->
<partition name="APP_b" id="2" type="data">
    <allocation_policy> sequential </allocation_policy>
    <filesystem_type> basic </filesystem_type>
    <size> 48318382080 </size> <!-- 48 GB = 45 GiB -->    
    <file_system_attribute> 0 </file_system_attribute>
    <allocation_attribute> 0x8 </allocation_attribute>
    <align_boundary> 16384 </align_boundary>
    <percent_reserved> 0 </percent_reserved>
    <unique_guid> APPUUID_b </unique_guid>
    <filename> APPFILE_b </filename>
</partition>

....
</device>

📌 If you use a different SSD, always use the actual value (num_sectors) returned by blockdev --getsz. 📌 Ensure the rootfs A/B partition size is divisible by 16384 (align_boundary).

3. Flashing Command

Use the following command to flash with A/B partitioning:

sudo ROOTFS_AB=1 ROOTFS_RETRY_COUNT_MAX=3 \
./tools/kernel_flash/l4t_initrd_flash.sh \
--external-device nvme0n1p1 \
-c tools/kernel_flash/flash_l4t_nvme_rootfs_ab.xml \
-p "-c bootloader/generic/cfg/flash_t234_qspi.xml" \
--showlogs nru50p internal

Parameter Explanation

Parameter	Description
ROOTFS_AB=1	Enables A/B rootfs partitioning
ROOTFS_RETRY_COUNT_MAX=3	Sets maximum retry count before marking a slot as bad
--external-device nvme0n1p1	Target device for rootfs (external NVMe SSD)
-c flash_l4t_nvme_rootfs_ab.xml	Custom partition layout XML file
-p "-c flash_t234_qspi.xml"	SPI QSPI config file for bootloader
--showlogs	Displays flashing log in real time
nru50p internal	Target board name and storage configuration

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4. After Flashing

Once flashing completes successfully:

• The system will boot from slot A (APP) by default.

• Slot B (APP_b) will remain as the inactive partition.

• You can verify the A/B slot status with nvbootctrl :

sudo nvbootctrl get-current-slot
sudo nvbootctrl get-number-slots
sudo nvbootctrl dump-slots-info

Example:

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5. Switching Slots (Optional)

If you need to switch the boot slot manually:

sudo nvbootctrl set-active-boot-slot 1
sudo reboot

The system will reboot into the B slot.

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6. Troubleshooting

• If partition table is not applied correctly → Double-check the total num_sectors in your XML matches your SSD capacity.

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7. Useful Commands

• Check partitions on target after boot:

lsblk

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✅ Summary: By using ROOTFS_AB=1 and a custom XML layout, you can create a reliable dual-slot system suitable for OTA updates or production deployment. If slot A fails after an update, the system can safely fall back to slot B, ensuring service continuity.