Re: consider dropping defrag of journals on btrfs

[Lennart Poettering <lennart@xxxxxxxxxxxxxx>]

On Mo, 08.02.21 22:13, Chris Murphy (lists@xxxxxxxxxxxxxxxxx) wrote:

> On Mon, Feb 8, 2021 at 7:56 AM Phillip Susi <phill@xxxxxxxxxxxx> wrote:
> >
> >
> > Chris Murphy writes:
> >
> > >> It sounds like you are arguing that it is better to do the wrong thing
> > >> on all SSDs rather than do the right thing on ones that aren't broken.
> > >
> > > No I'm suggesting there isn't currently a way to isolate
> > > defragmentation to just HDDs.
> >
> > Yes, but it sounded like you were suggesting that we shouldn't even try,
> > not just that it isn't 100% accurate.  Sure, some SSDs will be stupid
> > and report that they are rotational, but most aren't stupid, so it's a
> > good idea to disable the defragmentation on drives that report that they
> > are non rotational.
>
> So far I've seen, all USB devices report rotational. All USB flash
> drives, and any SSD in an enclosure.
>
> Maybe some way of estimating rotational based on latency standard
> deviation, and stick that in sysfs, instead of trusting device
> reporting. But in the meantime, the imperfect rule could be do not
> defragment unless it's SCSI/SATA/SAS and it reports it's rotational.

btrfs itelf has a knob declaring whether something is ssd or not ssd,
configurable via the mount option. Of course, one would bind any
higher level logic to that same thing, and thus make it btrfs' own
problem, or the admin's.

Lennart

--
Lennart Poettering, Berlin
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