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Message-ID: <CAHk-=wi+cVOE3VmJzN3C6TFepszCkrXeAFJY6b7bK=vV493rzQ@mail.gmail.com>
Date: Wed, 1 Nov 2023 12:23:44 -1000
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: Trond Myklebust <trondmy@...merspace.com>
Cc: Jan Kara <jack@...e.cz>, Chris Mason <clm@...com>,
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Subject: Re: [PATCH RFC 2/9] timekeeping: new interfaces for multigrain
timestamp handing
On Wed, Nov 1, 2023, 11:35 Trond Myklebust <trondmy@...merspace.com> wrote:
>
> My client writes to the file and immediately reads the ctime. A 3rd
> party client then writes immediately after my ctime read.
> A reboot occurs (maybe minutes later), then I re-read the ctime, and
> get the same value as before the 3rd party write.
>
> Yes, most of the time that is better than the naked ctime, but not
> across a reboot.
Ahh, I knew I was missing something.
But I think it's fixable, with an additional rule:
- when generating STATX_CHANGE_COOKIE, if the ctime matches the
current time and the ctime counter is zero, set the ctime counter to
1.
That means that you will have *spurious* cache invalidations of such
cached data after a reboot, but only for reads that happened right
after the file was written.
Now, it's obviously not unheard of to finish writing a file, and then
immediately reading the results again.
But at least those caches should be somewhat limited (and the problem
then only happens when the nfs server is rebooted).
I *assume* that the whole thundering herd issue with lots of clients
tends to be for stable files, not files that were just written and
then immediately cached?
I dunno. I'm sure there's still some thinko here.
Linus
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