Re: Limitations of rsnapshot-style backups (Was: Re: lazy old guy asks question)

[Dan Ritter <dsr@randomstring.org>]

David Christensen wrote: 
> On 9/2/25 06:05, Dan Ritter wrote:
> > David Christensen wrote:
> > > 
> > > a.  Set the ZFS backup file system property "dedup".  This will enable
> > > block-level de-duplication, which can de-duplicate data more than hard links
> > > alone.
> > 
> > This is generally not a good thing to recommend; one of the
> > authors of the system wrote a good article which should
> > definitely be read before turning on dedup:
> > https://despairlabs.com/blog/posts/2024-10-27-openzfs-dedup-is-good-dont-use-it/
> > 
> > -dsr-
> 
> 
> Thank you for pointing out the article.
> 
> 
> AIUI the thesis of the article is that "dedup is worse than useless on
> general purpose workloads" [section "Unique Entries", paragraph two].
> 
> 
> It would be interesting to know the OpenZFS developers' opinion of dedup for
> backup workloads, including special vdev class vs. dedup vdev class.


That is addressed at the end, and, in particular, you should
note the bit about reflinks... which for any workload which can
use it, provides a rsnapshot-style deduplication mechanism.