Hammer2 offline deduplication

Evert Mouw post at evert.net
Thu Nov 7 08:01:11 PST 2019

I'm not a DragonflyBSD user, so take my words not too seriously; I'm
following this list because I consider Hammer2 for future use for my
personal data.

Is there a CLI command that can be invoked to force online dedup? And
some method to allocate it more resources and see the progress?

For filesystem maintenance, offline operations are desirable. I once
deduped a btrfs partition (not containing critical data, of course) and
offline dedup did work pretty well.

Anyhow, just my 2c.


On Thu, Nov 07, 2019 at 09:31:51AM -0500, Justin Sherrill wrote:
>Are you looking for how it works technically, or how to operate it?
>The design document goes into the technical details of how it's set
>The design notes mention that, like you say, online dedup is on by
>default.  There's no utility to run it offline yet, though I don't
>know how much of a difference that makes in practice; online dedup may
>already do most of the work.   I'd be interested in statistics on
>On Wed, Nov 6, 2019 at 2:25 PM Mikhail Novosyolov
><m.novosyolov at rosalinux.ru> wrote:
>> Hi,
>> I have not been able to find documentation about how deduplication works
>> in Hammer2. I have only found that online deduplication is now on by
>> default, but there seems to be no description how it works.
>> Is it possible to make offline deduplication? I mean finding blocks with
>> the same checksum and making one block point to another instead of
>> spending space on disk twice? There are several utilities that do this
>> on BTRFS in Linux, for example https://github.com/Zygo/bees

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nor flesh-and-blood.  Persons... have opportunities, not rights, which they
use or do not use.
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