GNU bug report logs - #6004
[libvirt] [PATCH 3/4] Fix QEMU save/restore with block devices

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Message #5 received at submit <at> debbugs.gnu.org (full text, mbox):

From: Eric Blake <eblake <at> redhat.com>
To: "Daniel P. Berrange" <berrange <at> redhat.com>
Cc: libvir-list <at> redhat.com, bug-coreutils <bug-coreutils <at> gnu.org>
Subject: Re: [libvirt] [PATCH 3/4] Fix QEMU save/restore with block devices
Date: Thu, 22 Apr 2010 08:39:26 -0600

[Message part 1 (text/plain, inline)]

[adding bug-coreutils]

On 04/22/2010 04:37 AM, Daniel P. Berrange wrote:
>>> -    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

Coreutils comes with an extension 'dd status=noxfer' which silences some
(but not all) output to stderr, but you'd have to test whether you are
targetting coreutils' dd (if dd comes from somewhere else, like busybox,
you'll cause a syntax error that prevents dd from doing anything at all).

There was a patch submitted to coreutils [1] that would add
status=noinfo, but it is currently held up by copyright status and lack
of documentation.  Maybe I should revive that patch (or rather, write it
from scratch, to avoid copyright taint).  But even so, you are still up
against the issue of testing whether you are targetting new-enough dd.

[1] http://lists.gnu.org/archive/html/bug-coreutils/2010-02/msg00161.html

About the best you can portably do, then, is capture stderr, then check
the exit status of dd; if the exit status is 0, discard the captured
stderr; otherwise, pass the stderr on to the logfile:

foo | dd of=a seek=$n 2>b; st=$?; if test $st != 0; then cat b >&2; \
  fi && rm -f b && exit $st

-- 
Eric Blake   eblake <at> redhat.com    +1-801-349-2682
Libvirt virtualization library http://libvirt.org

[signature.asc (application/pgp-signature, attachment)]

Message #8 received at submit <at> debbugs.gnu.org (full text, mbox):

From: "Daniel P. Berrange" <berrange <at> redhat.com>
To: Eric Blake <eblake <at> redhat.com>
Cc: libvir-list <at> redhat.com, bug-coreutils <bug-coreutils <at> gnu.org>
Subject: Re: [libvirt] [PATCH 3/4] Fix QEMU save/restore with block devices
Date: Thu, 22 Apr 2010 15:53:37 +0100

On Thu, Apr 22, 2010 at 08:39:26AM -0600, Eric Blake wrote:
> [adding bug-coreutils]
> 
> On 04/22/2010 04:37 AM, Daniel P. Berrange wrote:
> >>> -    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
> 
> Coreutils comes with an extension 'dd status=noxfer' which silences some
> (but not all) output to stderr, but you'd have to test whether you are
> targetting coreutils' dd (if dd comes from somewhere else, like busybox,
> you'll cause a syntax error that prevents dd from doing anything at all).

Yep, just looked at status=noxfer, but there's not much point in it since
it still prints other stats unconditionally.

> 
> There was a patch submitted to coreutils [1] that would add
> status=noinfo, but it is currently held up by copyright status and lack
> of documentation.  Maybe I should revive that patch (or rather, write it
> from scratch, to avoid copyright taint).  But even so, you are still up
> against the issue of testing whether you are targetting new-enough dd.
> 
> [1] http://lists.gnu.org/archive/html/bug-coreutils/2010-02/msg00161.html
> 
> About the best you can portably do, then, is capture stderr, then check
> the exit status of dd; if the exit status is 0, discard the captured
> stderr; otherwise, pass the stderr on to the logfile:
> 
> foo | dd of=a seek=$n 2>b; st=$?; if test $st != 0; then cat b >&2; \
>   fi && rm -f b && exit $st

It really isn't worth the bother. A couple of lines of dd output in the
logfile is no harm to anything.

Daniel
-- 
|: Red Hat, Engineering, London    -o-   http://people.redhat.com/berrange/ :|
|: http://libvirt.org -o- http://virt-manager.org -o- http://deltacloud.org :|
|: http://autobuild.org        -o-         http://search.cpan.org/~danberr/ :|
|: GnuPG: 7D3B9505  -o-   F3C9 553F A1DA 4AC2 5648 23C1 B3DF F742 7D3B 9505 :|

Message #11 received at submit <at> debbugs.gnu.org (full text, mbox):

From: Jim Meyering <jim <at> meyering.net>
To: Eric Blake <eblake <at> redhat.com>
Cc: libvir-list <at> redhat.com, bug-coreutils <bug-coreutils <at> gnu.org>,
	"Daniel P. Berrange" <berrange <at> redhat.com>
Subject: Re: [libvirt] [PATCH 3/4] Fix QEMU save/restore with block devices
Date: Sun, 24 Mar 2013 04:03:22 +0100

tags 6004 + notabug
close 6004
thanks

Eric Blake wrote:
> [adding bug-coreutils]
>
> On 04/22/2010 04:37 AM, Daniel P. Berrange wrote:
>>>> - 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
>
> Coreutils comes with an extension 'dd status=noxfer' which silences some
> (but not all) output to stderr, but you'd have to test whether you are
> targetting coreutils' dd (if dd comes from somewhere else, like busybox,
> you'll cause a syntax error that prevents dd from doing anything at all).
>
> There was a patch submitted to coreutils [1] that would add
> status=noinfo, but it is currently held up by copyright status and lack
> of documentation.  Maybe I should revive that patch (or rather, write it
> from scratch, to avoid copyright taint).  But even so, you are still up
> against the issue of testing whether you are targetting new-enough dd.
>
> [1] http://lists.gnu.org/archive/html/bug-coreutils/2010-02/msg00161.html
...

As of coreutils-8.20,

  dd now accepts 'status=none' to suppress all informational output.

so I've closed this.

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