[ClusterLabs] Q: About a false negative of storage_mon

[Klaus Wenninger]

On Fri, Aug 5, 2022 at 9:30 AM Kazunori INOUE
<kazunori_inoue at newson.co.jp> wrote:
>
> On Tue, Aug 2, 2022 at 11:09 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.
> >
> > 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>
> >
>
> Thank you, everyone.
> I will create a patch using O_DIRECT.
> (I'm also interested in AIO, but I don't have a track record of using it,

Not saying you have to use it nor that it is a perfect reference implementation
but it is proven to work to a certain extent - and well - I know it ;-)

https://github.com/ClusterLabs/sbd/blob/main/src/sbd-md.c

Not sure as well if it is of much benefit for a usage pattern without
a persistent daemon.
And it is Linux proprietary. Unfortunately posix aio implementation seems
not to be using the kernel-api (yet).

What you could adopt from the sbd-code might be getting the block-size
from the device instead of assuming 512 bytes (what iirc the current code does).

Klaus

> so I'm going to see it off this time.)
>
> Kazunori INOUE
>
>
>
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