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Re: [libvirt] [PATCH 3/4] Fix QEMU save/restore with block devices



On Thu, Apr 22, 2010 at 11:36:06AM +0100, Daniel P. Berrange wrote:
> On Wed, Apr 21, 2010 at 11:02:33PM +0200, Daniel Veillard wrote:
> > On Wed, Apr 21, 2010 at 05:56:12PM +0100, Daniel P. Berrange wrote:
> > > The save process was relying on use of the shell >> append
> > > operator to ensure the save data was placed after the libvirt
> > > header + XML. This doesn't work for block devices though.
> > > Replace this code with use of 'dd' and its 'seek' parameter.
> > > 
> > > The qemuMonitorMigateToCommand() monitor API is used for both
> > > save/coredump, and migration via UNIX socket. We can't simply
> > > switch this to use 'dd' since this causes problems with the
> > > migration usage. Thus, create a dedicated qemuMonitorMigateToFile
> > > which can accept an filename + offset, and remove the filename
> > > from the current qemuMonitorMigateToCommand() API
> > > 
> > > * src/qemu/qemu_driver.c: Switch to qemuMonitorMigateToFile
> > >   for save and core dump
> > > * src/qemu/qemu_monitor.c, src/qemu/qemu_monitor.h,
> > >   src/qemu/qemu_monitor_json.c, src/qemu/qemu_monitor_json.h,
> > >   src/qemu/qemu_monitor_text.c, src/qemu/qemu_monitor_text.h: Create
> > >   a new qemuMonitorMigateToFile, separate from the existing
> > >   qemuMonitorMigateToCommand to allow handling file offsets
> > > ---
> > >  src/qemu/qemu_driver.c       |  172 +++++++++++++++++++++++-------------------
> > >  src/qemu/qemu_monitor.c      |   28 ++++++--
> > >  src/qemu/qemu_monitor.h      |    9 ++-
> > >  src/qemu/qemu_monitor_json.c |   35 ++++++++-
> > >  src/qemu/qemu_monitor_json.h |    9 ++-
> > >  src/qemu/qemu_monitor_text.c |   35 ++++++++-
> > >  src/qemu/qemu_monitor_text.h |    9 ++-
> > >  7 files changed, 201 insertions(+), 96 deletions(-)
> > > 
> > [...]
> > >  
> > > -    if (virAsprintf(&dest, "exec:%s >>%s 2>/dev/null", argstr, safe_target) < 0) {
> > > +    if (virAsprintf(&dest, "exec:%s | dd of=%s seek=%llub",
> > > +                    argstr, safe_target, offset) < 0) {
> > 
> >  hum %llu will be converted to the value and then 'b' is appended. But
> > my reading is that 'b' means block i.e. 512 bytes, and what we need is
> > skeep to offset bytes, i.e. use seek=%lluc since 'c' means characters,
> > or am I mistaken ?
> 
> Hmm, yes I think you are correct. Oddly QEMU was happy enough restoring
> from this.

I assume dd will just push at the end if the offset is greater than the
file size instead of creating a sparse file, so we end up with what we
expect :-)

Daniel

-- 
Daniel Veillard      | libxml Gnome XML XSLT toolkit  http://xmlsoft.org/
daniel veillard com  | Rpmfind RPM search engine http://rpmfind.net/
http://veillard.com/ | virtualization library  http://libvirt.org/


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