On Friday 22 August 2008, Phillip Lougher wrote: > > > > This looks very nice, but could use some comments about how the data is > > actually stored on disk. It took me some time to figure out that it actually > > allows to do tail merging into compressed blocks, which I was about to suggest > > you implement ;-). Cramfs doesn't have them, and I found that they are the > > main reason why squashfs compresses better than cramfs, besides the default > > block size, which you can change on either one. > > Squashfs has much larger block sizes than cramfs (last time I looked it > was limited to 4K blocks), and it compresses the metadata which helps to > get better compression. But tail merging (fragments in Squashfs > terminology) is obviously a major reason why Squashfs gets good compression. The *default* block size in cramfs is smaller than in squashfs, but they both have user selectable block sizes. I found the impact of compressed metadata to be almost zero. I hacked up a mksquashfs to avoid tail merging, and found that the image size for squashfs and cramfs is practically identical if you use the same block size and no tail merging. > The AXFS code is rather obscure but it doesn't look to me that it does > tail merging. The following code wouldn't work if the block in question > was a tail contained in a larger block. It assumes the block extends to > the end of the compressed block (cblk_size - cnode_offset). yes, I thought the same thing when I first read that code, and was about to send a lengthy reply about how it should be changed when I saw that it already does exactly that ;-). Arnd <>< -- To unsubscribe from this list: send the line "unsubscribe linux-embedded" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
