[file system] how to run squashfs on UBI

stay UBI Run on squashfs File is openwrt Common methods in the system ：

• rootfs function squashfs （ read-only ）
• roootfs_data function ubifs （ Reading and writing ）

because ubifs Is running on the UBI File systems on volumes , and squashfs Is running on the block device File system on top of , There is no intersection between the two , however UBI Provides the ability to ubi Features of creating read-only block devices on volumes (R/O block devices on top of UBI volumes), This makes squashfs Can also “ function ” stay ubi On the roll .

R/O block devices on top of UBI volumes

UBI Allow in UBI Create a read-only block device on the volume , This volume is intended for read-only 、 Block oriented file system , for example squashfs.
UBI Allow in UBI Create a block device on the volume , But there are the following restrictions ：

• read-only operation .
• Serialization I/O operation , But remember NAND The driver kernel has also serialized all I/O.

Despite these limitations , Block device for UBI It is still useful to mount a read-only regular file system on a volume . With squashfs For example , It can be used as NAND Lightweight read-only on the device rootfs. under these circumstances ,UBI Layer will handle low-level details , For example, bit reversal processing and wear equalization .

Realization principle

1. Based on any MTD equipment ,attach After creating 2 individual UBI volume (volume0 and volume1)
2. take squashfs Image storage volume0, take ubifs Image storage volume 1
3. be based on volume0 establish ubi block device, Use this block device mount squashfs
4. Use vloume 1 mount ubifs

Be careful : If you don't need to run ubifs, So just create one volume function squashfs It's OK .

demonstration

The following will demonstrate how to implement in ubi Run on squashfs：
Before doing the following , Need to know nand flash Some of the parameters . This is what I'm using nand flash Parameters of ：

• block size 256KB
• page size 4KB
• (UBI)LEB 248KB
• (UBI)PEB 256KB
• flash total size 4096 blocks
• flash min I/O size 4KB

Make squashfs

Make squashfs Need to use mksquashfs Tools , For instructions, see mksquashfs

establish fs The contents are as follows ：

$ ls
apps  this_is_squashfs.txt

Execute the following command ： according to fs directories creating squashfs, Output squashfs The image file is squashfs.img

mksquashfs fs/ squashfs.img -b 256k -nopad -noappend -root-owned -comp xz -processors 1

• -b Appoint flash One block Size
• -comp xz Specify the compression algorithm as xz
• -root-owned Of all files in the file system owner All are root
• -noappend Do not append to an existing file system
• -nopad Do not populate the file system with 4K Multiple
• -processors 1 Use 1 Processors build file systems

Make ubifs

Make squashfs Need to use mkfs.ubifs Tools , Please refer to establish UBIFS Mirror image

establish fs The contents are as follows ：

$ ls
apps  this_is_ubifs.txt

Execute the following command ： according to fs directories creating ubifs, Output ubifs The image file is ubifs.img

mkfs.ubifs -m 4KiB -e 248KiB -c 128  --space-fixup   --compr=lzo  --squash-uids -r fs -o ubifs.img

Make ubi Mirror image

Make ubi Mirroring requires ubinize Tools , Reference resources ubinize Tools

in addition ,ubinize The tool also needs a description ubi Configuration file of the volume ubinize.cfg：

[my_squashfs]
mode=ubi
vol_id=0
vol_type=dynamic
vol_name=my_squashfs
image=squashfs.img

[my_ubifs]
mode=ubi
vol_id=1
vol_type=dynamic
vol_name=my_ubifs
vol_flags=autoresize
image=ubifs.img

The above configuration file contains 2 Volumes ：

• Dynamic volume 0, Distribution for them ID 0 And named it my_squashfs; The contents of the volume are taken from squashfs.img ;
• Dynamic volume 1, Distribution for them ID 1 And named it my_ubifs; The contents of the volume are taken from ubifs.img;

my_squashfs The type of volume must be dynamic, Otherwise, it cannot be mounted later

Last , Execute the following command to generate ubi Mirror image ubi.image

ubinize -m 4KiB -p 256KiB ubinize.cfg -o ubi.image

burning ubi Mirror image

Generated in the previous step ubi.image Can write directly to flash, Use here ubiformat Tools for burning .

• burning ubi Mirror image

ubiformat /dev/mtd3 -y -f ubi.image

• attach

ubiattach -p /dev/mtd3

attach Then a new ubi Equipment and ubi volume .
Here I generate ubi2, And according to ubinize.cfg The configuration file automatically generates two volumes ：my_squashfs and myubifs.

$ ubinfo -a /dev/ubi2

Volume ID:   0 (on ubi2)
Type:        dynamic
Alignment:   1
Size:        1 LEBs (253952 bytes, 248.0 KiB)
State:       OK
Name:        my_squashfs
Character device major/minor: 506:1
-----------------------------------
Volume ID:   1 (on ubi2)
Type:        dynamic
Alignment:   1
Size:        111 LEBs (28188672 bytes, 26.8 MiB)
State:       OK
Name:        my_ubifs
Character device major/minor: 506:2

mount

about ubifs You can mount directly , Use /dev/ubi2_1 Just mount ：

mount -t ubifs /dev/ubi2_1 /test

View the files in the file system as follows ：

ls /test -l
drwxrwxr-x    2 root     root           224 Jun 22 14:00 app
-rw-rw-r--    1 root     root            16 Jun 22 14:00 this_is_ubifs.txt

about squashfs Further operations are needed ：

• Create a read-only block device
  ubiblock Tools are used to create or destroy ubi Block device , How to use it is as follows ：

Usage: ubiblock [-c,-r] <UBI volume node file name>
Example: 
	ubiblock -c /dev/ubi0_0
	ubiblock -r /dev/ubi0_0

be based on ubi2_0 establish ubi The commands of the block device are as follows ：

ubiblock -c /dev/ubi2_0

After execution , Will be in /dev Generate ubi Block device file

# ls /dev/ubiblock2* -l
brw-------    1 root     root      252,   1 Jun 22 14:38 /dev/ubiblock2_0

Using the above block device, you can mount squashfs 了

mount -t squashfs /dev/ubiblock2_0 /test1

View the files in the file system as follows ：

# ls test1/ -l
drwxrwxr-x    2 root     root            30 Jun 22 10:51 apps
-rw-rw-r--    1 root     root            47 Jun 22 13:59 this_is_squashfs.txt

Check the mount information ：

$ mount
/dev/ubi2_1 on /test type ubifs (rw,relatime,assert=read-only,ubi=2,vol=1)
/dev/ubiblock2_0 on /test1 type squashfs (ro,relatime)

Reference resources

• Squashfs 4.0 Filesystem
• mksquashfs
• SquashFS HOWTO
• UBIFS