Re: [Libguestfs] [PATCH v4 0/3] v2v: Add -o rhv-upload output mode.

On Wed, Mar 14, 2018 at 9:04 PM Richard W.M. Jones <rjones(a)redhat.com&gt; wrote: What is the use case for flushing ranges? I guess we will have one or few flushes per images. Looking at sync_file_range(2), it does not seem to be a safe way to flush: Warning This system call is extremely dangerous and should not be used in portable programs. None of these operations writes out the file's metadata. Therefore, unless the application is strictly performing overwrites of already-instantiated disk blocks, there are no guarantees that the data will be available after a crash. There is no user interface to know if a write is purely an overwrite. On file systems using copy-on-write semantics (e.g., btrfs) an overwrite of existing allocated blocks is impossible. When writing into preal‐ located space, many file systems also require calls into the block allocator, which this system call does not sync out to disk. This system call does not flush disk write caches and thus does not provide any data integrity on systems with volatile disk write caches. I can support the same size and offset arguments, and treat them as a hint if we can implement this safely in some future version. But I think providing only simple and safe way to flush is good enough for this context.