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



On Wed, Apr 21, 2010 at 03:03:43PM -0600, Eric Blake wrote:
> On 04/21/2010 10:56 AM, 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
> > 
> 
> > +++ b/src/qemu/qemu_driver.c
> > @@ -4789,6 +4789,7 @@ static int qemudDomainSaveFlag(virDomainPtr dom, const char *path,
> >      qemuDomainObjPrivatePtr priv;
> >      struct stat sb;
> >      int is_bdev = 0;
> > +    unsigned long long offset;
> 
> Should this be off_t instead of ull?
> 
> > @@ -1673,7 +1701,8 @@ int qemuMonitorJSONMigrateToCommand(qemuMonitorPtr mon,
> >          goto cleanup;
> >      }
> >  
> > -    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) {
> 
> Don't you still need to silence stderr, particularly since dd writes to
> stderr even on success? (2 instances)

I didn't want to silence stderr, because I want it to end up in the QEMU
logfile if anything goes wrong. So i really need  a way to make dd keep
quiet on success, rather than throwing away stderr

Daniel
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