はじめに

OpenWrtのrootfsをUSBメモリで拡張する方法でrootfsを拡張しましたが、いろいろ欲張ろうとするとメモリ不足になるので、更にUSBメモリでswapを確保することにしました。
ルータはGL.iNet GL-AR150を利用しています。

参考：

OSバージョンを確認します

root@OpenWrt:~# cat /etc/openwrt_release
DISTRIB_ID='OpenWrt'
DISTRIB_RELEASE='18.06.1'
DISTRIB_REVISION='r7258-5eb055306f'
DISTRIB_TARGET='ar71xx/generic'
DISTRIB_ARCH='mips_24kc'
DISTRIB_DESCRIPTION='OpenWrt 18.06.1 r7258-5eb055306f'
DISTRIB_TAINTS=''

必要なパッケージをインストールします

root@GL-AR150:~# opkg update
root@GL-AR150:~# opkg install kmod-usb-storage usbutils gdisk block-mount e2fsprogs kmod-fs-ext4
root@GL-AR150:~# reboot

USBメモリ(/dev/sda)をフォーマットします

#####現在のシステム設定を確認します

root@OpenWrt:~# df
Filesystem           1K-blocks      Used Available Use% Mounted on
/dev/root                 2560      2560         0 100% /rom
tmpfs                    30160       420     29740   1% /tmp
/dev/mtdblock5           12288      2196     10092  18% /overlay
overlayfs:/overlay       12288      2196     10092  18% /
tmpfs                      512         0       512   0% /dev

root@OpenWrt:~# dmesg | grep sd
[   16.101834] random: ubusd: uninitialized urandom read (4 bytes read)
[   16.109496] random: ubusd: uninitialized urandom read (4 bytes read)
[   16.201745] random: ubusd: uninitialized urandom read (4 bytes read)
[  156.982667] sd 0:0:0:0: [sda] 15633408 512-byte logical blocks: (8.00 GB/7.45 GiB)
[  156.991501] sd 0:0:0:0: [sda] Write Protect is off
[  156.995029] sd 0:0:0:0: [sda] Mode Sense: 43 00 00 00
[  157.001802] sd 0:0:0:0: [sda] Write cache: disabled, read cache: enabled, doesn't support DPO or FUA
[  157.036142]  sda: sda1
[  157.045232] sd 0:0:0:0: [sda] Attached SCSI disk

#####フォーマットを開始します

root@OpenWrt:~# gdisk /dev/sda
GPT fdisk (gdisk) version 1.0.3

Partition table scan:
  MBR: protective
  BSD: not present
  APM: not present
  GPT: present

Found valid GPT with protective MBR; using GPT.

Command (? for help): p
Disk /dev/sda: 15633408 sectors, 7.5 GiB
Model: Cruzer Fit      
Sector size (logical/physical): 512/512 bytes
Disk identifier (GUID): 6478B437-EECD-AA48-A2EC-56C50B396DFD
Partition table holds up to 128 entries
Main partition table begins at sector 2 and ends at sector 33
First usable sector is 34, last usable sector is 15633374
Partitions will be aligned on 2048-sector boundaries
Total free space is 2014 sectors (1007.0 KiB)

Number  Start (sector)    End (sector)  Size       Code  Name
   1            2048        15633374   7.5 GiB     8300  Linux filesystem

#####既存パーテイションを削除します

Command (? for help): d
Using 1

Command (? for help): p
Disk /dev/sda: 15633408 sectors, 7.5 GiB
Model: Cruzer Fit      
Sector size (logical/physical): 512/512 bytes
Disk identifier (GUID): 6478B437-EECD-AA48-A2EC-56C50B396DFD
Partition table holds up to 128 entries
Main partition table begins at sector 2 and ends at sector 33
First usable sector is 34, last usable sector is 15633374
Partitions will be aligned on 2048-sector boundaries
Total free space is 15633341 sectors (7.5 GiB)

Number  Start (sector)    End (sector)  Size       Code  Name

#####rootfs用パーティションを作成します

Command (? for help): n
Partition number (1-128, default 1): 
First sector (34-15633374, default = 2048) or {+-}size{KMGTP}: 
Last sector (2048-15633374, default = 15633374) or {+-}size{KMGTP}: +7G
Current type is 'Linux filesystem'
Hex code or GUID (L to show codes, Enter = 8300): 
Changed type of partition to 'Linux filesystem'

#####swap用パーティションを作成します

Command (? for help): n
Partition number (2-128, default 2): 
First sector (34-15633374, default = 14682112) or {+-}size{KMGTP}: 
Last sector (14682112-15633374, default = 15633374) or {+-}size{KMGTP}: 
Current type is 'Linux filesystem'
Hex code or GUID (L to show codes, Enter = 8300): 8200
Changed type of partition to 'Linux swap'

#####設定内容を保存します

Command (? for help): w

Final checks complete. About to write GPT data. THIS WILL OVERWRITE EXISTING
PARTITIONS!!

Do you want to proceed? (Y/N): Y
OK; writing new GUID partition table (GPT) to /dev/sda.
The operation has completed successfully.

#####rootfs用ファイル・システムを作成します

root@OpenWrt:~# mkfs.ext4 /dev/sda1
mke2fs 1.44.1 (24-Mar-2018)
/dev/sda1 contains a ext4 file system
	last mounted on /tmp/extroot/overlay on Thu Jan  1 09:00:13 1970
Proceed anyway? (y,N) y
Creating filesystem with 1835008 4k blocks and 458752 inodes
Filesystem UUID: 6c7a2efd-414b-4f7d-a0f1-1d24eabc69d8
Superblock backups stored on blocks: 
	32768, 98304, 163840, 229376, 294912, 819200, 884736, 1605632

Allocating group tables: done                            
Writing inode tables: done                            
Creating journal (16384 blocks): done
Writing superblocks and filesystem accounting information: done

#####swap用ファイル・システムを作成します

root@OpenWrt:~# mkswap /dev/sda2
Setting up swapspace version 1, size = 487042560 bytes

#####設定内容を確認します

root@OpenWrt:~# block info | grep "/dev/sd"
/dev/sda1: UUID="6c7a2efd-414b-4f7d-a0f1-1d24eabc69d8" VERSION="1.0" TYPE="ext4"
/dev/sda2: VERSION="1" TYPE="swap"

root@OpenWrt:~# block detect
config 'global'
	option	anon_swap	'0'
	option	anon_mount	'0'
	option	auto_swap	'1'
	option	auto_mount	'1'
	option	delay_root	'5'
	option	check_fs	'0'

config 'mount'
	option	target	'/mnt/sda1'
	option	uuid	'6c7a2efd-414b-4f7d-a0f1-1d24eabc69d8'
	option	enabled	'0'

config 'swap'
	option	device	'/dev/sda2'
	option	enabled	'0'

システム起動時に/dev/sda1と/dev/sda2を自動マウントするようにします

#####fstabを変更します

root@OpenWrt:~# block detect > /etc/config/fstab
root@OpenWrt:~# uci set fstab.@mount[0].enabled='1'
root@OpenWrt:~# uci set fstab.@swap[0].enabled='1'
root@OpenWrt:~# uci commit

#####再起動します

root@OpenWrt:~# reboot

#####自動マウントできました

root@OpenWrt:~# df
Filesystem           1K-blocks      Used Available Use% Mounted on
/dev/root                 2560      2560         0 100% /rom
tmpfs                    30160       420     29740   1% /tmp
/dev/mtdblock5           12288      2196     10092  18% /overlay
overlayfs:/overlay       12288      2196     10092  18% /
tmpfs                      512         0       512   0% /dev
/dev/sda1              7159288     32244   6743660   0% /mnt/sda1

root@OpenWrt:~# uci show fstab
fstab.@global[0]=global
fstab.@global[0].anon_swap='0'
fstab.@global[0].anon_mount='0'
fstab.@global[0].auto_swap='1'
fstab.@global[0].auto_mount='1'
fstab.@global[0].delay_root='5'
fstab.@global[0].check_fs='0'
fstab.@mount[0]=mount
fstab.@mount[0].target='/mnt/sda1'
fstab.@mount[0].uuid='6c7a2efd-414b-4f7d-a0f1-1d24eabc69d8'
fstab.@mount[0].enabled='1'
fstab.@swap[0]=swap
fstab.@swap[0].device='/dev/sda2'
fstab.@swap[0].enabled='1'

root@OpenWrt:~# block info
/dev/mtdblock4: UUID="9e82422c-eee4f4a6-d814e9ad-6c8c5a7c" VERSION="4.0" MOUNT="/rom" TYPE="squashfs"
/dev/mtdblock5: MOUNT="/overlay" TYPE="jffs2"
/dev/sda1: UUID="6c7a2efd-414b-4f7d-a0f1-1d24eabc69d8" VERSION="1.0" MOUNT="/mnt/sda1" TYPE="ext4"
/dev/sda2: VERSION="1" TYPE="swap"

/dev/sda1をrootfsに切り替え、/dev/sda2でswapを確保します

#####現在のrootfs(/overlay)を/mnt/sda1にコピーします

root@OpenWrt:~# tar -C /overlay -cvf - . | tar -C /mnt/sda1 -xf -

#####fstabを変更します

root@OpenWrt:~# block detect > /etc/config/fstab
root@OpenWrt:~# sed -i s/option$'\t'enabled$'\t'\'0\'/option$'\t'enabled$'\t'\'1\'/ /etc/config/fstab
root@OpenWrt:~# sed -i s#/mnt/sda1#/overlay# /etc/config/fstab
root@OpenWrt:~# cat /etc/config/fstab
config 'global'
	option	anon_swap	'0'
	option	anon_mount	'0'
	option	auto_swap	'1'
	option	auto_mount	'1'
	option	delay_root	'5'
	option	check_fs	'0'

config 'mount'
	option	target	'/overlay'
	option	uuid	'6c7a2efd-414b-4f7d-a0f1-1d24eabc69d8'
	option	enabled	'1'

config 'swap'
	option	device	'/dev/sda2'
	option	enabled	'1'

#####再起動します

root@OpenWrt:~# reboot

できました

root@OpenWrt:~# df
Filesystem           1K-blocks      Used Available Use% Mounted on
/dev/root                 2560      2560         0 100% /rom
tmpfs                    30160       412     29748   1% /tmp
/dev/sda1              7159288     36624   6739280   1% /overlay
overlayfs:/overlay     7159288     36624   6739280   1% /
tmpfs                      512         0       512   0% /dev
/dev/sda1              7159288     36624   6739280   1% /mnt/sda1

root@OpenWrt:~# free
             total       used       free     shared    buffers     cached
Mem:         60320      25112      35208        412       3780       7724
-/+ buffers/cache:      13608      46712
Swap:       475624          0     475624