海风游戏狂欢季-独家活动首发平台

Squashfs的设计是专门为一般的只读文件系统的使用而设计，它可应用于数据备份，或是系统资源紧张的电脑上使用。

最初版本的Squashfs采用 gzip 的数据压缩。版本 2.6.34 之后的Linux内核增加了对 LZMA[1] 和 LZO [2]压缩算法的支持，版本 2.6.38 的内核增加了对LZMA2的支持，该算法同时也是xz使用的压缩算法。

squashfs 系统支持以回环(loopback)的方式挂载，然后便可以访问其上的文件了，在访问这些文件时，它们就会被解压缩并装载在 RAM 中，而不需要将整个文件解压缩后才去访问其中的文件，这样一来访问速度就快多了。

1 内核squashfs相关配置

1.1 kernel配置

Linux中打开squashfs功能：

File systems

->Miscellaneous filesystems

->SquashFS 4.0 - Squashed file system support

->File decompression options (Decompress file data into an intermediate buffer)

->Decompressor parallelisation options (Single threaded compression)

->Squashfs XATTR support

->Include support for ZLIB compressed file systems

->Include support for LZ4 compressed file systems

->Include support for LZO compressed file systems

->Include support for XZ compressed file systems

->Include support for ZSTD compressed file systems

->Use 4K device block size?

->Additional option for memory-constrained systems

1.2 作为rootfs挂载

在bootargs上增加如下命令：

root=/dev/mtdblock20 ro rootfstype=squashfs rootwait

通过mount命令加载squashfs：

mount -t squashfs /dev/mtdblock20 /rootfs

2 Buildroot生成squashfs镜像

Buildroot配置如下：

Filesystem images

squashfs root filesystem

block size (128k)

pad to a 4K boundary

Compression algorithm (gzip)

extreme compression when available

使能squashfs后编译mksquashfs工具，使用方法如下：

SYNTAX:./output/host/bin/mksquashfs source1 source2 ... dest [options] [-e list of exclude

dirs/files]

Filesystem build options:

-tar read uncompressed tar file from standard in (stdin)

-no-strip act like tar, and do not strip leading directories

from source files

-tarstyle alternative name for -no-strip

-cpiostyle act like cpio, and read files from standard in (stdin)

-cpiostyle0 like -cpiostyle, but filenames are null terminated

-comp select compression--指定压缩算法。

Compressors available:

gzip (default)

lzma

lzo

lz4

xz

zstd

-b set data block to . Default 128 Kbytes--指定block大小。

Optionally a suffix of K or M can be given to specify

Kbytes or Mbytes respectively

...

Filesystem filter options:

...

Filesystem append options:

-noappend do not append to existing filesystem--创建全新文件系统。

-root-becomes when appending source files/directories, make the

original root become a subdirectory in the new root

called , rather than adding the new source items

to the original root

Mksquashfs runtime options:

-version print version, licence and copyright message

-exit-on-error treat normally ignored errors as fatal

-recover recover filesystem data using recovery file

-no-recovery don't generate a recovery file

-recovery-path use as the directory to store the recovery file

-quiet no verbose output

-info print files written to filesystem

-no-progress don't display the progress bar

-progress display progress bar when using the -info option

-throttle throttle the I/O input rate by the given percentage.

This can be used to reduce the I/O and CPU consumption

of Mksquashfs

-limit limit the I/O input rate to the given percentage.

This can be used to reduce the I/O and CPU consumption

of Mksquashfs (alternative to -throttle)

-processors Use processors. By default will use number of--工作时可使用的的核数。

processors available

-mem Use physical memory. Currently set to 969M

Optionally a suffix of K, M or G can be given to specify

Kbytes, Mbytes or Gbytes respectively

Miscellaneous options:

...

Compressors available and compressor specific options:--压缩算法的子选项，提供更精细的配置。

gzip (default)

-Xcompression-level

should be 1 .. 9 (default 9)

-Xwindow-size

should be 8 .. 15 (default 15)

-Xstrategy strategy1,strategy2,...,strategyN

Compress using strategy1,strategy2,...,strategyN in turn

and choose the best compression.

Available strategies: default, filtered, huffman_only,

run_length_encoded and fixed

lzma (no options)

lzo

-Xalgorithm

Where is one of:

lzo1x_1

lzo1x_1_11

lzo1x_1_12

lzo1x_1_15

lzo1x_999 (default)

-Xcompression-level

should be 1 .. 9 (default 8)

Only applies to lzo1x_999 algorithm

lz4

-Xhc

Compress using LZ4 High Compression

xz

-Xbcj filter1,filter2,...,filterN

Compress using filter1,filter2,...,filterN in turn

(in addition to no filter), and choose the best compression.

Available filters: x86, arm, armthumb, powerpc, sparc, ia64

-Xdict-size

Use as the XZ dictionary size. The dictionary size

can be specified as a percentage of the block size, or as an

absolute value. The dictionary size must be less than or equal

to the block size and 8192 bytes or larger. It must also be

storable in the xz header as either 2^n or as 2^n+2^(n+1).

Example dict-sizes are 75%, 50%, 37.5%, 25%, or 32K, 16K, 8K

etc.

zstd

-Xcompression-level

should be 1 .. 22 (default 15)

在fs/squashfs/squashfs.mk中定义了生成squashfs镜像流程：

ROOTFS_SQUASHFS_ARGS = \

-noappend \

-processors $(PARALLEL_JOBS) \

-b $(call qstrip,$(BR2_TARGET_ROOTFS_SQUASHFS_BS)) \

$(call qstrip,$(BR2_TARGET_ROOTFS_SQUASHFS_COMP_OPTS))

ifeq ($(BR2_TARGET_ROOTFS_SQUASHFS_PAD),)

ROOTFS_SQUASHFS_ARGS += -nopad

endif

ifeq ($(BR2_TARGET_ROOTFS_SQUASHFS4_LZ4),y)

ROOTFS_SQUASHFS_ARGS += -comp lz4

else ifeq ($(BR2_TARGET_ROOTFS_SQUASHFS4_LZO),y)

ROOTFS_SQUASHFS_ARGS += -comp lzo

else ifeq ($(BR2_TARGET_ROOTFS_SQUASHFS4_LZMA),y)

ROOTFS_SQUASHFS_ARGS += -comp lzma

else ifeq ($(BR2_TARGET_ROOTFS_SQUASHFS4_XZ),y)

ROOTFS_SQUASHFS_ARGS += -comp xz

else ifeq ($(BR2_TARGET_ROOTFS_SQUASHFS4_ZSTD),y)

ROOTFS_SQUASHFS_ARGS += -comp zstd

else

ROOTFS_SQUASHFS_ARGS += -comp gzip

endif

define ROOTFS_SQUASHFS_CMD

$(HOST_DIR)/bin/mksquashfs $(TARGET_DIR) $@ $(ROOTFS_SQUASHFS_ARGS)

endef

参考文档：《[OpenWrt Wiki] The OpenWrt Flash Layout》、《Squash Fs Comparisons - eLinux.org》、《Understanding SquashFS and How to Mount a SquashFS Filesystem | Baeldung on Linux》。