Re: [Qemu-devel] [PATCH v3] block: Add support for Secure Shell (ssh) block device.

[Richard W.M. Jones]

On Tue, Mar 26, 2013 at 05:23:03PM +0100, Stefan Hajnoczi wrote:
> On Tue, Mar 26, 2013 at 02:21:39PM +0000, Richard W.M. Jones wrote:
> > On Tue, Mar 26, 2013 at 10:38:46AM +0000, Richard W.M. Jones wrote:
> > > +    /* According to the docs, this just updates a field in the
> > > +     * sftp_handle structure, so there is no network traffic and it
> > > +     * cannot fail.
> > > +     */
> > > +    libssh2_sftp_seek64(s->sftp_handle, offset);
> > 
> > While I was looking at the performance of qemu+ssh vs sshfs, I noticed
> > that the line above has a catastrophic effect on performance, since it
> > causes all readahead data to be thrown away.  Ooops.  I'm reworking
> > the patch to make seeking more intelligent.
> 
> Should libssh2 be smarter and preserve the readahead when offsets are
> contiguous?
> 
> It makes sense to fix in QEMU for older libssh2 binaries, but the real
> fix might be for libssh2 to be smart.

It's quite tricky to fix at the libssh2 API level ...  However I sent
a preliminary patch upstream:

  http://www.libssh2.org/mail/libssh2-devel-archive-2013-03/0019.shtml

Rich.

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