Online migration of arbitrary filesystems, possible?

[danielhilst@gmail.com (Daniel Hilst)]

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?


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