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

["Richard W.M. Jones" <rjones@redhat.com>]

On Fri, Mar 22, 2013 at 01:04:55PM +0000, Richard W.M. Jones wrote:
> I got it working with Curl, patch attached.
> 
> However there are multiple issues (these are mainly notes for myself):
> 
> (1) libcurl cannot read the size of the file.  I had to hard-code
> this.  This is probably just a shortcoming of libcurl (libssh2/sftp
> itself can read the size of files).  Will try to work on a patch for
> upstream.

After my holiday and in the cold of day I've had a long look at the
SFTP implementation in libcurl.

  https://github.com/bagder/curl/blob/master/lib/ssh.c

It's implemented as a huge state machine and simply implementing (1)
above is problematic (I believe we would have to reopen the connection
after our call to curl_easy_perform).

The larger issue is that the qemu curl block driver doesn't support
writes.  Now these could in theory be added.  Indeed curl does support
"random access" writes, although AFAICT you have to open a new
connection each time you want to seek backwards, and you can't read
and write over the same connection (so you'd need >= 1 connections for
reading and another >= 1 connections for writing).

I think I will continue with the pure libssh2-based block driver,
making it support all the missing features discussed earlier:

  http://www.mail-archive.com/qemu-devel@nongnu.org/msg161997.html

plus of course non-blocking AIO.

I think this way we'll end up with a much more robust, reliable and
easier to debug ssh implementation in qemu.

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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