Arch Linux

lightstream

Member

From: Britain

Registered: 2011-10-30

Posts: 64

fstrim - what does it actually do?

From what I read, I got the impression that fstrim tells the drive firmware which blocks are no longer in use by the OS. However, after running it on my root filesystem, the results have confused me.

Here's the drive usage reported by df:

Filesystem      Size  Used Avail Use% Mounted on
/dev/nvme1n1p2  457G  415G   20G  96% /

And here's the output of fstrim:

# fstrim -v /
/: 42.7 GiB (45901381632 bytes) trimmed

How come this number is so much larger than the free space actually on the disk? (df reports the same amount of free space after running fstrim as it did before)

mpan

Member

Registered: 2012-08-01

Posts: 1,360

Website

Re: fstrim - what does it actually do?

Did you take into account space reserved for root? Unless you have used a different value, it’s 5% by default and it fits perfectly:

42.7 ≈ 20 + 5% · 457

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seth

Member

Registered: 2012-09-03

Posts: 60,756

Re: fstrim - what does it actually do?

Seen https://en.wikipedia.org/wiki/Trim_(computing) ?

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lightstream

Member

From: Britain

Registered: 2011-10-30

Posts: 64

Re: fstrim - what does it actually do?

Did you take into account space reserved for root? Unless you have used a different value, it’s 5% by default and it fits perfectly:

42.7 ≈ 20 + 5% · 457

No I didn't, that'll be what it is, thank you! TIL