git: HAMMER - Add live dedup sysctl and support

Ilya Dryomov idryomov at
Tue Jan 4 02:15:48 PST 2011

On Tue, Jan 04, 2011 at 10:00:18AM +0100, Thomas Nikolajsen wrote:
> Thank you!
> This looks really interesting, I will have to play with it right away.

Bear in mind that while it basically works it is marked experimental -
extensive testing as still ahead (obscure races, etc).  But any feedback
is very welcome!

> The commit message is a bit short IMHO, but code has extensive comments;
> I was enlightened quite a bit by the one below.

I wanted Matt to write commit message, but he refused ;)

> How is the relation between online (aka live) and offline HAMMER dedup?
> Is typical use to run both if online is used?

The thing is that as the quoted comment says there is this on-media
requirement, which along with simplifying code tremendously limits its
use cases (I mean the number of situations where duplicate data will be
dedup'ed).  The primary use case is cp and cpdup of files and directory
hierarchies.  If you want to get maximum of its abilities set sysctl
vfs.hammer.live_dedup=2, but note that it will slightly impact the
normal write path (performance wise).

As for the relation, both of them can and should be used together.  Live
dedup can be turned on at all times, while offline dedup should be run
periodically to pickup all the leftovers.  In combination this
arrangement gives us a full-fledged deduplication support in HAMMER
without major complications in the implementation.

> Is offline HAMMER dedup always more efficient (disk space wise)?
> Will online HAMMER dedup dedup data between PFSs (offline is per PFS, right)?

Yes, offline is per-PFS.  Online is fs-wide, that is it will dedup data
between PFSs (cp pfs1/a pfs2/b will be dedup'ed).

> What are your plans/thoughts to further enhance HAMMER dedup?

Apart from testing and closing races in live dedup:

1) The main issue with both offline and online dedup is reblocker.  Under
certain (rare) circumstances it may partially re-dedup dedup ;)  So
reblocker has to be made aware of dedup'ed data, but it is pretty a
separate project.

2) 'hammer dedup-everything' directive - fs-wide (as opposed to per-PFS)
offline dedup.  Actually I consider per-PFS separation more of a feature
than a drawback, but for people who think they may have possible
duplicates across PFSs it will be useful.

3) Per-file (and possibly per-directory) nodedup flag.

4) Make live dedup cache size a tunable (for now I think I'll just make
it a sysctl, but it clearly has to scale automatically).

This is all I can remember of the top of my head, any thoughts and
comments are welcome.



> ..
> +/************************************************************************
> + *                                            LIVE DEDUP                                                             *
> + ************************************************************************
> + *
> + * HAMMER Live Dedup (aka as efficient cp(1) implementation)
> + *
> + * The dedup cache is operated in a LRU fashion and destroyed on
> + * unmount, so essentially this is a live dedup on a cached dataset and
> + * not a full-fledged fs-wide one - we have a batched dedup for that.
> + * We allow duplicate entries in the buffer cache, data blocks are
> + * deduplicated only on their way to media. By default the cache is
> + * populated on reads only, but it can be populated on writes too.
> + *
> + * The main implementation gotcha is on-media requirement - in order for
> + * a data block to be added to a dedup cache it has to be present on
> + * disk. This simplifies cache logic a lot - once data is laid out on
> + * media it remains valid on media all the way up to the point where the
> + * related big block the data was stored in is freed - so there is only
> + * one place we need to bother with invalidation code.
> + */

More information about the Kernel mailing list