Online migration of arbitrary filesystems, possible?

[Daniel Hilst]

On 29-03-2013 17:28, Greg Freemyer wrote:
> 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
>
I can do this with btrfs snapshots, I think LVM2 can do this with 
snapshots too... I was searching for something like that, but using a 
kind of VFS hack

So I want to know if is possible, in theory, then I can start write 
something like that

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