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

Eric Blake eblake at redhat.com
Thu Nov 1 12:51:12 UTC 2012

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.

Eric Blake   eblake at redhat.com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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