[ClusterLabs] Q: About a false negative of storage_mon

[Klaus Wenninger]

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.
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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