Online migration of arbitrary filesystems, possible?

[Greg Freemyer]

On Fri, Mar 29, 2013 at 4:09 PM, Daniel Hilst <danielhilst at gmail.com> wrote:
> 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?

Are you looking for a pre-existing solution?  Or you want to write something?

Greg