[ClusterLabs] Antw: [EXT] Re: Q: About a false negative of storage_mon

[Klaus Wenninger]

On Wed, Aug 3, 2022 at 4:02 PM Ulrich Windl
<Ulrich.Windl at rz.uni-regensburg.de> wrote:
>
> >>> Klaus Wenninger <kwenning at redhat.com> schrieb am 03.08.2022 um 15:51 in
> Nachricht
> <CALrDAo074Vd77biwe9NP6J7k2V9DkNg1YfApjrUHMm0-6L8L_g at mail.gmail.com>:
> > On Tue, Aug 2, 2022 at 4:10 PM Ken Gaillot <kgaillot at redhat.com> wrote:
> >>
> >> On Tue, 2022-08-02 at 19:13 +0900, 井上和徳 wrote:
> >> > Hi,
> >> >
> >> > Since O_DIRECT is not specified in open() [1], it reads the buffer
> >> > cache and
> >> > may result in a false negative. I fear that this possibility
> >> > increases
> >> > in environments with large buffer cache and running disk-reading
> >> > applications
> >> > such as database.
> >> >
> >> > So, I think it's better to specify O_RDONLY|O_DIRECT, but what about
> >> > it?
> >> > (in this case, lseek() processing is unnecessary.)
> >> >
> >> > # I am ready to create a patch that works with O_DIRECT. Also, I
> >> > wouldn't mind
> >> > # a "change to add a new mode of inspection with O_DIRECT
> >> > # (add a option to storage_mon) while keeping the current inspection
> >> > process".
> >> >
> >> > [1]
> >> >
> >
> https://github.com/ClusterLabs/resource-agents/blob/main/tools/storage_mon.c#
>
> > L47-L90
> >> >
> >> > Best Regards,
> >> > Kazunori INOUE
> >>
> >> I agree, it makes sense to use O_DIRECT when available. I don't think
> >> an option is necessary.
> >
> > Might as well be interesting to adjust block-size/alignment to the
> > device.
> > Another consideration could be to on top directly access the block-layer
> > using aio.
>
> Again AIO is POSIX; it depends on the implementation what it really does.

Wasn't speaking of the Linux POSIX AIO implementation in userspace
(guess that is still the case) but what is available as syscalls
(io_submit, io_setup, io_cancel, io_destroy, io_getevents) that is afaik
Linux proprietary and can't be wrapped into the Posix interface.

>
> > Both is being done in sbd (storage-based-death) and yes it as well
> > adds Linux specific stuff that might have to be conditional for other OSs.
> >
> > Klaus
> >
> >>
> >> However, O_DIRECT is not available on all OSes, so the configure script
> >> should detect support. Also, it is not supported by all filesystems, so
> >> if the open fails, we should retry without O_DIRECT.
> >> --
> >> Ken Gaillot <kgaillot at redhat.com>
> >>
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