Suppose that I have two big filesystems, dest fs and source fs.. I want to copy whole source fs to dest fs, but I need to keep source filesystem online I can't mount source fs read only and copy.. The idea is, mount both filesystems "together", and make write/read operations go on this way Read operations: 1. See if data is already on dest fs, 2. If is then read data and bright back to caller (lets call this cold read) 3. If is not, then read file from source fs, put it on page cache, and change the backstorage of that page.. 3.1 So when this page get dirty or too old, it will be writed to dest fs The problem here is, I need to remap every data and metadata on step 3 (inodes and stuff), I think that read only data, isn't writed to disk, so in case of old page, this page would be freed from memory and not write to dest fs.. so I need read only pages to be forced dirty.. Write operations: 1. Write data to dest fs 2. Mark data as present on dest fs Also I need to know when copy is finished, so I need a tool to crosscheck both filesystems and say what was copied and what wasn't, and give me some percent of remaining unsynced data. When I get 100% of data in sync I can umount source filesystem and use its storage for other propose. The go here is move data online, where I can't bring data offline.. I'm a kernel newbie, I have just read a lot about page cache and VSF, but this seems possible, right? -- Follow the white rabbit!