Re: Are 'guix gc' stats exaggerated?

[Efraim Flashner]

On Sun, May 26, 2024 at 01:13:45PM -0700, Felix Lechner via Development of GNU 
Guix and the GNU System distribution. wrote:
> Hi,
> 
> Today I ran 'guix gc' on equipment with an ext4 root partition.  It had
> these space characteristics beforehand:
> 
>     Filesystem  Size       Used      Avail     Use% Mounted on
>     /dev/dm-3   309047680  157252980 138126064  54% /
> 
> or for human eyes:
> 
>     /dev/dm-3   295G        150G      132G      54% /
> 
> After the run, the drive showed:
> 
>     /dev/dm-3   309047680   88267956 207111088  30% /
> 
> or for human eyes:
> 
>     /dev/dm-3   295G         85G      198G      30% /
> 
> By my math, about 65.8 GiB were recovered.
> 
> When 'guix gc' was done, it announced:
> 
>     [184389 MiB] deleting '/gnu/store/...'
>     deleting `/gnu/store/trash'
>     deleting unused links...
>     note: currently hard linking saves 59224.03 MiB
>     guix gc: freed 110,649.49 MiBs
> 
> Seeing the 184389 MiB number, or 180 GiB, already made me suspicious.
> It exceeded my drive usage by 30 GiB.  Even the more conservative 110649
> MiB "freed," however, are off by a mile. That would have been 108 GiB,
> or 42 GiB more than the space actually recovered.
> 
> Am I looking at those numbers the wrong way?  Thanks!

As your store grows larger the inherent deduplication from the
guix-daemon approaches a 3:1 file deduplication ratio.  If two files are
the same then they are hardlinked to the same actual block on the drive
and you save some space.

I have found that if you switch to btrfs and add zstd (level 3)
compression then you get about another 2:1 on top of that, for around
5.5:1.

-- 
Efraim Flashner   <efraim@flashner.co.il>   רנשלפ םירפא
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