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Date: Mon, 25 Mar 2013 14:12:56 +0100
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From: Stefan Hajnoczi <stefanha@gmail.com>
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Subject: Re: [Qemu-devel] [PATCH] block/curl: Add support for Secure Shell
 (ssh/sftp) block device
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To: "Richard W.M. Jones" <rjones@redhat.com>
Cc: qemu-devel <qemu-devel@nongnu.org>

On Mon, Mar 25, 2013 at 1:32 PM, Richard W.M. Jones <rjones@redhat.com> wrote:
> 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.

Fair enough.

Stefan