树莓派4 U盘引导启动
一、硬件镜像准备
1、准备Micro SD卡(8 or 16GB) 和 USB3.0 U盘1个（大于8G）
2、准备镜像
树莓派官方Raspbian系统，官网免费下载：点这里
树莓派镜像
3、准备烧录工具
目前balenaEtcher支持各个平台，推荐使用给这个烧录(下载地址：点这里)
烧录工具
4、烧录
解压树莓派镜像得到2020-02-13-raspbian-buster.img文件，打开烧录工具选择image、分别选择2个存储设备进行2次烧录，完成后得到2个启动盘。
二、树莓派4设置
1、将sd卡和U盘分别接到设备上如图
15882969324743.jpg
15882969256677.jpg
2、用SSH连接 或者 树莓派终端登陆
3、运行blkid查看设备信息，如下面，可以看到新版树莓派4 SD卡和外置存储器PARTUUID相同，所以不需要修改cmdline.txt文件
pi@raspberrypi:~ $ sudo lsblk -o name,label,partuuid
NAME LABEL PARTUUID
sda
├─sda1 boot ea7d04d6-01
└─sda2 rootfs ea7d04d6-02
mmcblk0
├─mmcblk0p1 boot ea7d04d6-01
└─mmcblk0p2 rootfs ea7d04d6-02
4、运行如下命令删除sd卡中rootfs分区
pi@raspberrypi:~ $ sudo fdisk /dev/mmcblk0
Welcome to fdisk (util-linux 2.33.1).
Changes will remain in memory only, until you decide to write them.
Be careful before using the write command.
Command (m for help): d
Partition number (1,2, default 2): 2
Partition 2 has been deleted.
Command (m for help): w
The partition table has been altered.
Failed to remove partition 2 from system: Device or resource busy
The kernel still uses the old partitions. The new table will be used at the next reboot.
Syncing disks.
pi@raspberrypi:~ $ sudo reboot
然后输入‘d’，再选择2，删除第二分区之后，输入‘w’。保存退出之后重启树莓派。
5、删除U盘中boot分区（也可以调过）
当系统启动之后执行以下命令：
pi@raspberrypi:~ $ sudo fdisk /dev/sda
Welcome to fdisk (util-linux 2.33.1).
Changes will remain in memory only, until you decide to write them.
Be careful before using the write command.
Command (m for help): d
Partition number (1,2, default 2): 1
Partition 1 has been deleted.
Command (m for help): w
The partition table has been altered.
Failed to remove partition 1 from system: Device or resource busy
The kernel still uses the old partitions. The new table will be used at the next reboot.
Syncing disks.
pi@raspberrypi:~ $ sudo reboot
然后输入‘d’，再选择1，删除第一分区之后，输入‘w’。保存退出之后重启树莓派。
pi@raspberrypi:~ $ sudo blkid
/dev/mmcblk0p1: LABEL_FATBOOT="boot" LABEL="boot" UUID="6341-C9E5" TYPE="vfat" PARTUUID="ea7d04d6-01"
/dev/sda2: LABEL="rootfs" UUID="80571af6-21c9-48a0-9df5-cffb60cf79af" TYPE="ext4" PARTUUID="ea7d04d6-02"
/dev/mmcblk0: PTUUID="ea7d04d6" PTTYPE="dos"
此时内存卡中只有个启动分区，外置储存设备中只有rootfs系统分区了。
6、调整文件系统大小
pi@raspberrypi:~ $ sudo fdisk /dev/sda
Welcome to fdisk (util-linux 2.33.1).
Changes will remain in memory only, until you decide to write them.
Be careful before using the write command.
Command (m for help): p
Disk /dev/sda: 28.7 GiB, 30765219840 bytes, 60088320 sectors
Disk model: Ultra Fit
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: dos
Disk identifier: 0xea7d04d6
Device Boot Start End Sectors Size Id Type
/dev/sda2 532480 7397375 6864896 3.3G 83 Linux
Command (m for help): q
记录下 532480 起始 sector 后面resize时候使用。
接下来调整分区大小，步骤如下：
pi@raspberrypi:~ $ sudo fdisk /dev/sda
Welcome to fdisk (util-linux 2.33.1).
Changes will remain in memory only, until you decide to write them.
Be careful before using the write command.
Command (m for help): d
Selected partition 2
Partition 2 has been deleted.
Command (m for help): n
Partition type
p primary (0 primary, 0 extended, 4 free)
e extended (container for logical partitions)
Select (default p): p
Partition number (1-4, default 1): 2
First sector (2048-60088319, default 2048): 532480
Last sector, +/-sectors or +/-size{K,M,G,T,P} (532480-60088319, default 60088319):
Created a new partition 2 of type 'Linux' and of size 28.4 GiB.
Partition #2 contains a ext4 signature.
Do you want to remove the signature? [Y]es/[N]o: n
Command (m for help): w
The partition table has been altered.
Syncing disks.
pi@raspberrypi:~ $ sudo reboot
重启后连接设备，查看分区大小
pi@raspberrypi:~ $ df -h
Filesystem Size Used Avail Use% Mounted on
/dev/root 3.2G 2.9G 98M 97% /
devtmpfs 1.8G 0 1.8G 0% /dev
tmpfs 2.0G 0 2.0G 0% /dev/shm
tmpfs 2.0G 8.6M 1.9G 1% /run
tmpfs 5.0M 4.0K 5.0M 1% /run/lock
tmpfs 2.0G 0 2.0G 0% /sys/fs/cgroup
/dev/mmcblk0p1 253M 54M 199M 22% /boot
tmpfs 391M 0 391M 0% /run/user/1000
看到/dev/root分区大小还没有重新调整，接下来使用resize2fs 命令来调整
pi@raspberrypi:~ $ sudo resize2fs /dev/sda2
resize2fs 1.44.5 (15-Dec-2018)
Filesystem at /dev/sda2 is mounted on /; on-line resizing required
old_desc_blocks = 1, new_desc_blocks = 2
The filesystem on /dev/sda2 is now 7444480 (4k) blocks long.
pi@raspberrypi:~ $ df -h
Filesystem Size Used Avail Use% Mounted on
/dev/root 28G 2.9G 24G 11% /
devtmpfs 1.8G 0 1.8G 0% /dev
tmpfs 2.0G 0 2.0G 0% /dev/shm
tmpfs 2.0G 8.6M 1.9G 1% /run
tmpfs 5.0M 4.0K 5.0M 1% /run/lock
tmpfs 2.0G 0 2.0G 0% /sys/fs/cgroup
/dev/mmcblk0p1 253M 54M 199M 22% /boot
tmpfs 391M 0 391M 0% /run/user/1000
经过上面的步骤 可以看到/dev/root分区大小已经调整过来了。
到此树莓派4可以通过usb的rootfs启动，速度大大提升！
