Re: How does BTRFS compression influence existing and new snapshots?

["Thorsten Schöning" <tschoening@am-soft.de>]

Guten Tag Zygo Blaxell,
am Freitag, 28. Mai 2021 um 21:36 schrieben Sie:

> Nothing happens to the snapshots.  'fi defrag' will make new, possibly
> compressed (but possibly not compressed) duplicate copies of the data
> in the listed files.  These copies will use separate storage space from
> the snapshots.

Thanks for clearing things up, that what's I expected already.

> Each individual extent written on the filesystem contains its own
> independent copy of compression parameters[...]

This brings me to the following in the wiki, not sure if it's worth an
additional thread:

> There is a simple decision logic: if the first portion of data being
> compressed is not smaller than the original, the compression of the
> file is disabled[...]

https://btrfs.wiki.kernel.org/index.php/Compression#What_happens_to_incompressible_files.3F

If (de)compression methods are managed by extents always anway, is the
statement in the wiki really true that compression of a whole FILE is
disabled if the first portion can't be compressed? Or does the quoted
sentence refer to EXTENTS instead of whole FILES instead?

"btrfs-compsize" prints the following for one of my directories, which
means that at least some parts of the file are compressed, others are
not.

> Processed 230 files, 754137 regular extents (754137 refs), 3 inline.
> Type       Perc     Disk Usage   Uncompressed Referenced
> TOTAL       87%      163G         187G         187G
> none       100%      130G         130G         130G
> lzo         57%       32G          56G          56G

So does this really mean that I was simply lucky because "the first
portion" of the file could be compressed? If that wouldn't the case,
the whole file would be uncompressed even though other parts of the
file might be compressed pretty well?

Referrring to the FILE in the wiki instead of EXTENTS doesn't make too
much sense to me currently.

Mit freundlichen Grüßen

Thorsten Schöning

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Für Rückfragen stehe ich Ihnen sehr gerne zur Verfügung.

Mit freundlichen Grüßen

Thorsten Schöning


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