Date: Sun, 14 Jan 2024 17:38:52 +0100 From: Olivier Certner <olce@freebsd.org> To: Rick Macklem <rick.macklem@gmail.com> Cc: "Lyndon Nerenberg (VE7TFX/VE6BBM)" <lyndon@orthanc.ca>, freebsd-current@freebsd.org Subject: Re: noatime on ufs2 Message-ID: <4014880.cjyAsbXg9l@ravel> In-Reply-To: <CAM5tNy4HuHb=GpQXvE4h2NC8_azOwDS3H7wY3UK%2BcE07owPr3A@mail.gmail.com> References: <ZZqmmM-6f606bLJx@int21h> <a5e38bfbabd37084@orthanc.ca> <CAM5tNy4HuHb=GpQXvE4h2NC8_azOwDS3H7wY3UK%2BcE07owPr3A@mail.gmail.com>
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From: Olivier Certner <olce@freebsd.org>
To: Rick Macklem <rick.macklem@gmail.com>
Subject: Re: noatime on ufs2
Date: Sun, 14 Jan 2024 17:38:52 +0100
Message-ID: <4014880.cjyAsbXg9l@ravel>
MIME-Version: 1.0
Hi Rick,
> I do not have a strong opinion w.r.t. atime, but I do believe that
> changing the default would be a POLA violation.
While I value POLA very highly, at the same time I do not consider it a sac=
rosanct principle that must be followed in every possible circumstances. T=
here are many different ways and levels of being amazed, and varying counte=
rparties to gain in exchange, so there cannot be any absolute interpretatio=
n of it. Moreover, the stricter you are in general, the more you are pushi=
ng the project towards fossilization. It's true that there are lots of mec=
hanisms to allow both backwards compatibility and evolution in lots of diff=
erent cases, but they come at a cost which is increased complexity, amount =
of code and configuration, which has to be taken into account as well.
Here, I do not think activating 'noatime' by default would be a significant=
violation of POLA. On the contrary, I think that almost nobody will notic=
e it, so there barely will be any amazement. Why? First case: The user/ad=
min doesn't care, so is using the default. Most of them will never ever us=
e 'atime' for any purposes. Some will try to use if a few times, discoveri=
ng on occasion that they cannot because something messed up with them (if '=
atime' is the default) or because they are not maintainted (if 'noatime' is=
the default). Second case: The user/admin cares, and wants/needs to avoid=
the extra I/O, so decides to uses 'noatime'. These people won't even noti=
ce a change, since they are using 'noatime' already. Third case: The user/=
admin cares, and wants the access times to be updated. If he's using an ex=
plicit 'atime', it's the same as the previous case. If, as is likely, he's=
not explicit, he will notice the change as some of his scenarios will star=
t to fail, with more or less bad consequences. I don't think this is really=
different than lots of changes the project has gone through. The very imp=
ortant thing, but the only one, we would have to do is publicizing this par=
t correctly ("if you're relying on access times, be sure to change your mou=
nt options, and possibly configure your auto-mounting applications and/or o=
ther mount helpers so that 'atime' is explicitly enabled.").
I address other POLA-related points raised by Mark Millard in a direct resp=
onse to his mail.
=20
> Please look at this email thread, where the opinion w.r.t. atime
> seemed quite different:
> freebsd-hackers@ Oct. 5, 2023
> Subject: copy_file_range() doesn't update the atime of an empty file
>=20
> I'd put a url here, but gmail always puts the subject line in here when
> I copy/paste the url?
No problem, I found it online.
I only re-subscribed to hackers@ a few months ago after discovering I had b=
een seemingly unsubscribed for a while without knowing it.
=20
> Basically I did not think that updating the infd's atime when copy_file_r=
ange()
> did not actually copy any data, but the collective disagreed, so I patched
> the NFSv4 client. (I do not know if markj@'s patch did get committed).
> They also collectively thought that Linux did a poor job w.r.t. atime.
I completely support the view that, if a copy realized through VOP_READ() u=
pdates the access time, so should a copy realized through copy_file_range()=
=2E That is arguably also an application of POLA, but in fact here is much=
more: A matter of correctness.
However I fail to see how that thread, as a whole, has any connection with =
the discussion we are having. Could you please elaborate?
Thanks and regards.
=2D-=20
Olivier Certner
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