Online migration of arbitrary filesystems, possible?

[Valdis.Kletnieks at vt.edu]

On Mon, 01 Apr 2013 17:50:43 -0300, Daniel Hilst said:

> > Any reason you can't just 'rsync /source-fs /dest-fs'?
> >
> because I can't use dest-fs while rsynching

Sure you can. You just have to remember to pay attention to race
conditions - if you create foo/bar.dat on the dest and then rsync
wants to copy over a foo/bar.tar from the source, things will go
poorly.

However, if you wanted to write to the dest while doing your sync,
you'll have that issue no matter *what* method you use to do it.

>> 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

You may want to look for 'overlayfs' and 'unionfs', which may provide
you the function you need. (Note there's several different patchsets
calling themselves 'unionfs').

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