Deduplication tools

Deduplication tools

[Marat Khalili]

After reading this maillist for a while I became a bit more cautious 
about using various BTRFS features, so decided to ask just in case: is 
it safe to use out-of-band deduplication tools 
<https://btrfs.wiki.kernel.org/index.php/Deduplication>, and which of 
them are considered more stable/mainstream? Also, won't running these 
tools exacerbate often mentioned stability/performance problems with 
too-many-snapshots? Any first-hand experience is very welcome.


--

With Best Regards,
Marat Khalili

Re: Deduplication tools

[Austin S. Hemmelgarn]

On 2017-04-13 07:06, Marat Khalili wrote:
> After reading this maillist for a while I became a bit more cautious
> about using various BTRFS features, so decided to ask just in case: is
> it safe to use out-of-band deduplication tools
> <https://btrfs.wiki.kernel.org/index.php/Deduplication>, and which of
> them are considered more stable/mainstream? Also, won't running these
> tools exacerbate often mentioned stability/performance problems with
> too-many-snapshots? Any first-hand experience is very welcome.
As a general rule, as long as you're careful, you generally shouldn't 
have many issues.  duperemove is the tool I would suggest for just 
generic deduplication on BTRFS, as I know the developer is still active 
and generally does a good job of getting bugs fixed.  It may well make 
performance problems with large numbers of snapshots worse, but probably 
not by as much as you think (unless you have huge amounts of duplicate 
data).  Keep in mind also that batch deduplication can take a very long 
time to do.

As a general rule though, if you're storing data that's consistently 
structured and organized, you may want to consider doing a custom tool 
to make the process more efficient.  Any generic deduplication tool is 
generally going to be pretty slow on large amounts of data, but 
depending on what the data is and how it's organized, it may be possible 
to determine duplicate data more efficiently than the block hashing 
method used by most deduplication tools.

Re: Deduplication tools

[Duncan]

Marat Khalili posted on Thu, 13 Apr 2017 14:06:07 +0300 as excerpted:

> After reading this maillist for a while I became a bit more cautious
> about using various BTRFS features, so decided to ask just in case: is
> it safe to use out-of-band deduplication tools
> <https://btrfs.wiki.kernel.org/index.php/Deduplication>, and which of
> them are considered more stable/mainstream? Also, won't running these
> tools exacerbate often mentioned stability/performance problems with
> too-many-snapshots? Any first-hand experience is very welcome.

Dedupe works by reflinking, which is how snapshots work too, so yes, 
it'll trigger the same general issues.

However, while snapshots will reflink /everything/ in the snapshot, one 
could hope dedupe won't find /that/ many duplicates and thus won't reflink 
at the scale that snapshotting does.  Tho of course if it doesn't find 
at least some reasonable number of duplicates, it's hardly worth the 
trouble to run.

So I'd say it may be worth doing, but do be aware of the scale of dedupe 
reflinking.  What I'd do here is consider that and then count it as 
roughly another snapshot or two, or whatever, depending on how much 
dedupeing you actually get.

-- 
Duncan - List replies preferred.   No HTML msgs.
"Every nonfree program has a lord, a master --
and if you use the program, he is your master."  Richard Stallman