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. :-)

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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