Re: [libvirt] [PATCH v2] iohelper: fsync() at the end

On 11/01/2012 06:33 AM, Michal Privoznik wrote: > Currently, when we are doing (managed) save, we insert the > iohelper between the qemu and OS. The pipe is created, the > writing end is passed to qemu and the reading end to the > iohelper. It reads data and write them into given file. However, > with write() being asynchronous data may still be in OS > caches and hence in some (corner) cases, all migration data > may have been read and written (not physically though). So > qemu will report success, as well as iohelper. However, with > some non local filesystems, where ENOSPACE is polled every X > time units, we may get into situation where all operations > succeeded but data hasn't reached the disk. And in fact will > never do. Therefore we ought sync caches to make sure data > has reached the block device on remote host. > --- > > + /* If we are on shared FS ensure all data is written as some > + * FSs may do writeback caching or polling for ENOSPC or any > + * other magic that local FS does not.*/ > + if (virStorageFileIsSharedFS(fdoutname) && (fdatasync(fdout) < 0)) { > + virReportSystemError(errno, _("unable to fsync %s"), fdoutname); > + goto cleanup; > + } I don't feel comfortable with that - we should do the fdatasync everywhere, not just network devices. It's better not to second-guess which file systems have which behaviors.