Re: [PATCH v3] btrfs: add BTRFS_IOC_GET_CSUMS ioctl

[David Sterba <dsterba@suse.cz>]

On Wed, Apr 15, 2026 at 02:56:49PM +0100, Mark Harmstone wrote:
> >> When using the --reflink option added in btrfs-progs v6.16.1, we can forgo
> >> reading the data entirely, resulting a ~2200% speed-up on the same test
> >> (128s to 6s).
> > 
> > Repeated mkfs is a specific use case, normaly it happens just once but I
> > understand that for preparing various images the time savings are
> > significiant.
> > 
> > Once this ioctl is available I think the deduplication tools will make
> > use of it as an initial filter before actually doing the real
> > deduplication of extents.
> 
> IIRC the mathematics of the birthday paradox is that for an n-bit hash, 
> you need 2^(n/2) sectors to have a 50% chance of a collision.
> 
> For the 32-bit hashes, that means a 50% chance of a collision every 256 
> MB. For the 256-bit hashes, that's 2^140 bytes... whatever that works 
> out as. So the dedupe logic would no doubt be that for larger hash sizes 
> GET_CSUMS itself will be sufficient.

It's used as a hint in the initial filter (e.g. in
https://github.com/lakshmipathi/dduper), the final deduplication does
full byte compariison (memcmp() inside vfs_dedupe_file_range_compare()).