I've upgraded to 4.1 release, it have great feature "Pass discards", that now 
can be used without vdsm hooks,
After upgrade I've tested it with NFS 4.1 storage, exported from netapp, but 
unfortunately found out, that
it's not working, after some investigation, I've found, that NFS 
implementation(even 4.1) in Centos 7
doesn't support sparse files and fallocate(FALLOC_FL_PUNCH_HOLE), that quemu 
uses for file storage, it was
added only in kernel 3.18, and sparse files is also announced feature of 
upcoming NFS4.2,
sparsify also not working on this data domains(runs, but nothing happens).

This test also shows, that FALLOC_FL_PUNCH_HOLE not working, it was executed on 
centos ovirt host with mounted nfs share:
# truncate -s 1024 test1
# fallocate -p -o 0 -l 1024 test1
fallocate: keep size mode (-n option) unsupported

Is there any plans to backport this feature to node-ng, or centos? or we should 
wait for RHEL 8?
NFS is more and more popular, so discards is VERY useful feature.
I'm also planning to test fallocate on latest fedora with 4.x kernel and 
mounted nfs.

Thanks for your work!


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