[tahoe-dev] BackupDB proposal

[Aleksandr Milewski]

On May 29, 2008, at 12:12 PM, Brian Warner wrote:
> It would be really cool if the filesystem were to provide us with a  
> reliable
> indicator of whether the file had changed: a version number, or a  
> strong hash
> of the contents. Maybe HFS+ or ZFS or reiser4 or one of the other  
> fancy new
> ones could provide this feature.
>
> I haven't measured it, but I believe that Apple's Time Machine does  
> a full
> backup of my home system (about 50GB) in maybe 2 or 3 minutes when  
> nothing
> has changed, so I must assume that they have a fast way to decide  
> that a file
> is probably unchanged without actually reading the contents. It  
> would be an
> interesting experiment to modify a file without changing its size,  
> then set
> the mtime back to its original value, then see if Time Machine  
> notices.

It's called FSEvents. I haven't dug into the dev docs, but there's a  
nice overview in the Ars review of Leopard: http://arstechnica.com/reviews/os/mac-os-x-10-5.ars/7

-Z