Hi,
Op 2011-08-07 16:19, Ryusuke Konishi schreef:
At present, nilfs does not have such per-file pointer to past
versions, so actually it's not optimal for file history lookups.
We may be able to do indirectly by adding a new field to on-disk inode
to keep a checkpoint number in which the inode lastly changed.
However, the new field must be updated every time garbage collector
thins out the previous version, -- this may complicate things
unacceptably because the current nilfs is designed so that it never
overwrites in-use blocks except super blocks.
Very interesting, thanks for your explanation. I agree that updating
such a field to a previous checkpoint should not be overwritten. A
solution would be to not update the field when the garbage collector
runs. The result is that a pointer to a previous checkpoint could point
to a non-existing checkpoint. That should not be a problem, because it
is known which checkpoints do exist, so a simple "if ( (
previous-checkpoint == X ) and ( X not element of available checkpoints
) ) then previous-checkpoint = NULL" would do. That would enable easier
checks for previous versions, although if there is some chechpoint
series like "1 2 3 4" and some file exists in all four of those, and if
you remove checkpoint 2, then this system will never find the oldest
version in checkpoint 1. But such a scenario is unlikely because the
garbage collector removes the oldest checkpoints first. Also, this
solution doesn't need to be perfect to be better than nothing at all.
Just my 2 cents, for I don't have any filesystem development experience.
Regards, Evert
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