This is a second attempt at mainlining Squashfs. The first attempt was way way back in early 2005 :-) Since then the filesystem layout has undergone two major revisions, and the kernel code has almost been completely rewritten. Both of these were to address the criticisms made at the original attempt. Summary of changes: 1. Filesystem layout is now 64-bit, in theory filesystems and files can be 2^64 in size. 2. Filesystem is now fixed little-endian. 3. "." and ".." are now returned by readdir. 4. Sparse files are now supported. 5. Filesystem is now exportable (NFS etc.). 6. Datablocks up to 1 Mbyte are now supported. Codewise all of the packed bit-fields and the swap macros have been removed in favour of aligned structures and in-line swapping using leXX_to_cpu(). The code has also been extensively restructured, reformatted to kernel coding standards and commented. Previously there was resistance to the inclusion of another compressed filesystem when Linux already had cramfs. There was pressure for a strong case to be made for the inclusion of Squashfs. Hopefully the case for the inclusion of other compressed filesystems has now already been answered over the last couple of years, however, it is worth listing the features of Squashfs over cramfs, which is still the only read-only compressed filesystem in mainline. Max filesystem size: cramfs 16 Mbytes, Squashfs 64-bit filesystem Max filesize: cramfs 16 Mbytes, Squashfs 64-bit filesystem Block size: cramfs 4K, Squashfs default 128K, max 1Mbyte Tail-end packing: cramfs no, Squashfs yes Directory indexes: cramfs no, Squashfs yes Compressed metadata: cramfs no, Squashfs yes Hard link support: cramfs no, Squashfs yes Support for "." and ".." in readdir: cramfs no, Squashfs yes Real inode numbers: cramfs no, Squashfs yes. Cramfs gives device inodes, fifo and empty directories the same inode of 1! Exportable filesystem (NFS, etc.): cramfs no, Squashfs yes Active maintenance: cramfs no (it is listed as orphaned, probably no active work for years), Squashfs yes Sorry for the list formatting, but many email readers are very unforgiving displaying tabbed lists and so I avoided them. For those that want hard performance statistics http://tree.celinuxforum.org/CelfPubWiki/SquashFsComparisons gives a full comparison of the performance of Squashfs against cramfs, zisofs, cloop and ext3. I made these tests a number of years ago using Squashfs 2.1, but they are still valid. In fact the performance should now be better. Cramfs is a limited filesystem, it's good for some embedded users but not now much else, its layout and features hasn't changed in the eight years+ since its release. Squashfs, despite never being in mainline, has been actively developed for over six years, and in that time has gone through four layout revisions, each revision improving compression and performance where limitations were found. For an often dismissed filesystem, Squashfs has advanced features such as metadata compression and tail-end packing for greater compression, and directory indexes for faster dentry operations. Despite not being in mainline, it is widely used. It is packaged by all major distributions (Ubuntu, Fedora, Debian, SUSE, Gentoo), it is used on most LiveCDs, it is extensively used in embedded systems (STBs, routers, mobile phones), and notably is used in such things as Splashtop and the Amazon Kindle. Anyway that's my case for inclusion. If any readers want Squashfs mainlined it's probably now a good time to offer support! There are 16 patches in the patch set, and the patches are against the latest linux-next tree (linux 2.6.27-next-20081016). Finally, I would like to acknowledge the financial support of the Consumer Embedded Linux Forum (CELF). They've made it possible for me to spend the last four months working full time on this mainlining attempt. Phillip -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
