Re: [Qemu-devel] [PATCH] block: Add support for Secure Shell (ssh) block device.

[Richard W.M. Jones]

On Mon, Mar 25, 2013 at 03:36:22PM +0100, Kevin Wolf wrote:
> You don't have a bdrv_co_flush_to_disk, what does this mean? Is every
> write immediately flushed to the disk, is the driver unsafe by design,
> or is this just a missing implementation detail?

Initially there is no .bdrv_co_flush_to_disk because this patch is
just a proof of concept.

However it does seem likely that a true flush-to-disk operation is not
possible with sftp.  Assuming this is supposed to guarantee that the
bits have been committed to disk, I don't see anything in the sftp
protocol that supports that guarantee.  It seems to be left up to the
implementation to do whatever it wants.

Here's the protocol v3 as implemented by OpenSSH:

  http://tools.ietf.org/html/draft-ietf-secsh-filexfer-02

and here's the latest version v6 (not implemented by anyone AFAIK):

  http://tools.ietf.org/html/draft-ietf-secsh-filexfer-13

if you think you can see anything that I've missed ...

Also I grepped over the source of OpenSSH and there is no call to
sync(2), fsync(2) or fdatasync(2).

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Fedora Windows cross-compiler. Compile Windows programs, test, and
build Windows installers. Over 100 libraries supported.
http://fedoraproject.org/wiki/MinGW