Re: time-shifting

[Evert Mouw <post-yfx6isLFVcpeoWH0uzbU5w@public.gmane.org>]

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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