VDSM copy functions with preallocated disks cause severe slowdowns on NFS v3 storage domains

Issue

• Performing a data copy function of a preallocated volume, such as creating a template, results in a slowdown of all i/o to/from the NFS server in question. The NFS file systems are V3, with the default mount options provided by RHV.

• On the SPM host, MiscFileReadException read timeouts in VDSM and high repoStats 'delay' times are seen, Storage Domains become 'INVALID', and VM's become unresponsive.

• All storage domains on the same NFS server are affected.

• This can be replicated by manually running;

  # qemu-img convert -p -t none -T none -f raw <src path> -O raw -o preallocation=falloc <dest path>

• Removing "-o preallocation=falloc" circumvents the problem, but the result is a sparse file instead of a preallocated file.