> 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 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). A c_block is the unit that gets compressed. It can contain multiple c_nodes. The c_block can be PAGE_SIZE to 4GB in size, in theory :) The c_nodes can be 1B to PAGE_SIZE. in any alignment. I pack many tails as c_nodes in a c_block. >>> + max_len = cblk_size - cnode_offset; >>> + len = max_len > PAGE_CACHE_SIZE ? PAGE_CACHE_SIZE : >>> max_len; >>> + src = (void *)((unsigned long)cblk0 + cnode_offset); >>> + memcpy(pgdata, src, len); > > Perhaps the AXFS authors could clarify this? The memcpy in question copies a c_node to the page. The len is either the max length of a c_node and size of the buffer I'm copying to (PAGE_CACHE_SIZE) or it is the difference between the beginning of the c_node in the c_block and the end of the c_block, whichever is smaller. The confusion is probably because of the fact that this copies extra crap to the page for tails. -- 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
