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List:       fedora-devel-list
Subject:    Re: Fedora 32 System-Wide Change proposal: Enable fstrim.timer by default
From:       Lennart Poettering <mzerqung () 0pointer ! de>
Date:       2019-12-20 17:21:34
Message-ID: 20191220172134.GA2059 () gardel-login
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On Fr, 20.12.19 18:11, Louis Lagendijk (louis@fazant.net) wrote:

> On Fri, 2019-12-20 at 17:46 +0100, Lennart Poettering wrote:
> >
> > Or let me ask this differently: the "discard" mount option of various
> > kernel file systems, what does it differently than what this new
> > fedora feature is supposed to do?
> >
> fstrim does the discard once a week (or whenever it it triggered),
> discard as a mount option does trigger discard when a block is freed.
> Depending on the drive it may actually slow down IO as the SSD will
> need more time to finish the IO. Doing an fstrim leaves the processing
> to the SDD. That was the argument years ago. I don't know if this is

Hmm? if the if the fs enqeues the trimming or userspace does, it's
always the SSD that executes it...

> still true for modern SSD's. For older SSDs fstrim would stil be the
> safer option. And automatic trimming is long overdue in my opinion.

So if trimming is slow, that's still no reason to let userspace pick a
time for it. Sounds like the kernel fs should have discard=lazy or
discard=5min or so. Which would enqueue a trim run automatically after
the last IO after some delay.

Still not grokking why to do this in userspace.

--
Lennart Poettering, Berlin
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