> >> We do the same for all of our "legacy" operating system backups.
> Take
> >> a snapshot then do an rsync and an excellent way of maintaining
> >> incremental backups for those.
> >
> >
> > Magic rsync options used:
> >
> >   -a --inplace --no-whole-file --delete-excluded
> >
> > This causes rsync to overwrite the file blocks in place rather than
> > writing to a new temporary file first.  As a result, zfs COW produces
> > primitive "deduplication" of at least the unchanged blocks (by
> writing
> > nothing) while writing new COW blocks for the changed blocks.
> If I understand your use case correctly (the application overwrites
> some blocks with the same exact contents), ZFS will ignore these "no-

I think he meant to rely on rsync here to do in-place updates of files and
only for changed blocks with the above parameters (by using rsync's own
delta mechanism). So if you have a file a and only one block changed rsync
will overwrite on destination only that single block.

> op" writes only on recent Open ZFS (illumos / FreeBSD / Linux) builds
> with checksum=sha256 and compression!=off.  AFAIK, Solaris ZFS will COW
> the blocks even if their content is identical to what's already there,
> causing the snapshots to diverge.
> See https://www.illumos.org/issues/3236 for details.

This is interesting. I didn't know about it.
Is there an option similar to verify=on in dedup or does it just assume that
"checksum is your data"?

Robert Milkowski

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