[Gluster-devel] How commonly applications make use of fadvise?

[Niels de Vos]

On Fri, Aug 11, 2017 at 05:50:47PM +0530, Ravishankar N wrote:
> 
> 
> On 08/11/2017 04:51 PM, Niels de Vos wrote:
> > On Fri, Aug 11, 2017 at 12:47:47AM -0400, Raghavendra Gowdappa wrote:
> > > Hi all,
> > > 
> > > In a conversation between me, Milind and Csaba, Milind pointed out
> > > fadvise(2) [1] and its potential benefits to Glusterfs' caching
> > > translators like read-ahead etc. After discussing about it, we agreed
> > > that our performance translators can leverage the hints to provide
> > > better performance. Now the question is how commonly applications
> > > actually provide hints? Is it something that is used quite frequently?
> > > If yes, we can think of implementing this in glusterfs (probably
> > > kernel-fuse too?). If no, there is not much of an advantage in
> > > spending our energies here. Your inputs will help us to prioritize
> > > this feature.
> > If functionality like this is available, we would add support in
> > libgfapi.so as well. NFS-Ganesha is prepared for consuming this
> > (fsal_obj_ops->io_advise), so applications running on top of NFS will
> > benefit. I failed to see if the standard Samba/vfs can use it.
> > 
> > A quick check in QEMU does not suggest it is used by the block drivers.
> > 
> > I don't think Linux/FUSE supports fadvise though. So this is an
> > oppertunity for a Gluster developer to get their name in the Linux
> > kernel :-) Feature additions like this have been done before by us, and
> > we should continue where we can. It is a relatively easy entry for
> > contributing to the Linux kernel.
> 
> To me it looks like fadvise (mm/fadvise.c) affects only the linux page cache
> behavior and is decoupled from the filesystem itself. What this means for
> fuse  is that the  'advise' is only to the content that the fuse kernel
> module has stored in that machine's page cache.  Exposing it as a FOP would
> likely involve adding a new fop to struct file_operations that is common
> across the entire VFS and likely  won't fly with the kernel folks. I could
> be wrong in understanding all of this. :-)

Thanks for checking! If that is the case, we need a good use-case to add
a fadvise function pointer to the file_operations. It is not impossible
to convince the Linux VFS developers, but it would not be as trivial as
adding it to FUSE only (but that requires the VFS infrastructure to be
there).

Niels


> Regards,
> Ravi
> > > [1] https://linux.die.net/man/2/fadvise
> > As well as local man-pages for fadvise64/posix_fadvise.
> > 
> > Showing that we have support for this, suggests that the filesystem
> > becomes more mature and gains advanced features. This should impress
> > users and might open up more interest for certain (HPC?) use-cases.
> > 
> > Thanks,
> > Niels
> > 
> > 
> > > regards,
> > > Raghavendra
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