You are ASSUMING that the new filesystem requires lessthan or equal amount
of metadata. This is NOT always true. A conversion of a full EXT2 to Riserfs
would fail simply because there is no free space to expand the needed
additional overhead.
Going in the other direction usually is possible (again, depending on the
filesystem) but there are exceptions... Try converting an EXT2 to DosFS.
In place. And maintain a recoverable state when aborted.
Not gonna happen.
Too much depends on what the target filesystem is, and what it may require.
Consider another - switching to an extent filesystem... If the datablocks
don't move, then you need MORE extents than the current indirect pointers.
And each extent is LARGER than the indirect pointers.
Then you have to compress/condense the extents (requiring shuffling data
blocks around to reduce the number of extents). Each requires free space
to do it's work, and the amount of free blocks is not the same.
Faster to do a copy. more reliable too. and recoverable.
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