6:19 p.m.
On Mon, 4 Jan 2016, Peter Krempa wrote:
On Thu, Dec 31, 2015 at 16:34:49 +1100, Michael Chapman wrote:
> Wait for a block job event after the job has reached 100% only if
> exactly one of the BLOCK_JOB and BLOCK_JOB_2 callbacks were successfully
> registered.
>
> If neither callback was registered, then no event will ever be received.
> If both were registered, then any user-supplied path is guaranteed to
> match one of the events.
>
> Signed-off-by: Michael Chapman <mike(a)very.puzzling.org>
> ---
>
> I have found that even a 2.5 second timeout isn't always sufficiently
> long for QEMU to flush a disk at the end of a block job.
>
> I hope I've understood the code properly here, because as far as I can
> tell the comment I'm removing in this commit isn't right. The path the
> user supplies *has* to be either the <source file='name'/> or
<target
> dev='name'/> exactly in order for the disk to be matched, and these are
> precisely the two strings used by the two events.
>
> tools/virsh-domain.c | 24 +++++++++++++-----------
> 1 file changed, 13 insertions(+), 11 deletions(-)
In addition to Andrea's review:
>
> diff --git a/tools/virsh-domain.c b/tools/virsh-domain.c
> index edbbc34..60de9ba 100644
> --- a/tools/virsh-domain.c
> +++ b/tools/virsh-domain.c
> @@ -1942,18 +1942,20 @@ virshBlockJobWait(virshBlockJobWaitDataPtr data)
> goto cleanup;
> }
>
> - /* since virsh can't guarantee that the path provided by the user will
> - * later be matched in the event we will need to keep the fallback
> - * approach and claim success if the block job finishes or vanishes. */
> - if (result == 0)
> - break;
> + /* if either event could not be registered we can't guarantee that the
> + * path provided by the user will be matched, so keep the fallback
> + * approach and claim success if the block job finishes or vanishes */
The new statement is not true. If the user provides a filesystem path
different from what is in the definition but matching the same volume [1]
the callback will still return the path present in the configuration and
thus might not ever match and the job would hang forever.
[1] e.g. /path/to/image and /path/to/../to/image are equivalent but
won't be equal using strcmp. The problem is even more prominent if you
mix in some symlinks.
As far I can tell the block job won't even be able to be identified if the
user specifies a different path than the one in the domain XML.
At present, the only implementation of the virGetBlockJobInfo API
is qemuDomainGetBlockJobInfo. The disk definition is found in the call
chain:
qemuDomainGetBlockJobInfo
-> virDomainDiskByName
-> virDomainDiskIndexByName
and virDomainDiskIndexByName only does STREQ tests to match the supplied
path against the <source> or <target> elements.
So at present, the disk path supplied by the user of virGetBlockJobInfo
has to be *exactly* the source path or the target name, and these are
precisely the two strings used in the two events.
The only safe way to allow different-but-equivalent paths to match the one
disk definition would be record the device+inode numbers in the disk
definition. We can't simply follow symlinks to resolve the path, since the
symlinks could have changed since the domain was started -- e.g.
/path/to/../to/image may now be equivalent to /path/to/image, but
/path/to/image may not be the same as what it was when the domain was
started.
So on that basis I think we have to settle for requiring the
virGetBlockJobInfo path to match the disk definition exactly.
> + if (data->cb_id2 < 0 || data->cb_id2 < 0) {
> + if (result == 0)
> + break;
>
> - /* for two-phase jobs we will try to wait in the synchronized phase
> - * for event arrival since 100% completion doesn't necessarily mean
that
> - * the block job has finished and can be terminated with success */
> - if (info.end == info.cur && --retries == 0) {
> - ret = VIR_DOMAIN_BLOCK_JOB_READY;
> - goto cleanup;
> + /* only wait in the synchronized phase for event arrival if
> + * either callback was registered */
> + if (info.end == info.cur &&
> + ((data->cb_id2 < 0 && data->cb_id2 < 0) ||
--retries == 0)) {
> + ret = VIR_DOMAIN_BLOCK_JOB_READY;
> + goto cleanup;
> + }
> }
NACK to this approach, if there was a way how this ugly stuff could be
avoided or deleted I'd already do so.
Peter
OK. At any rate, I do think 2.5 seconds is not enough. On a hypervisor
with a large amount of memory I was able to generate block jobs that would
take 10-15 seconds to fully flush out to disk.
- Michael