[ClusterLabs] Q: About a false negative of storage_mon

[Ken Gaillot]

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>