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Re: [libvirt] [PATCH 1/2] qemu: don't log error for missing optional sources on stats

On 12/11/18 2:34 AM, Nikolay Shirokovskiy wrote:
> On 11.12.2018 01:05, John Ferlan wrote:
>> $SUBJ:
>> 'storage sources'
>> On 11/12/18 7:58 AM, Nikolay Shirokovskiy wrote:
>>> Every time we call all domain stats for inactive domain with
>>> unavailable source we get error message in logs. It's a bit noisy.
>> Are there ones in particular?
> Like this one:
> qemuOpenFileAs:3324 : Failed to open file '/path/to/optinal/disk': No such file or directory

So this is the one that perhaps is bothersome to the degree of we're
ignoring that it doesn't exist, but then again the domain is not active,
so does it really matter.

>>> While it's arguable whether we need such message or not for mandatory
>>> disks we would like not to see messages for optional disks. Let's
>> Filtering for the 'startupPolicy = optional' for domain stats (with
>> empty/unavailable local source) would seem to be a "new use" perhaps not
>> totally in line with the original intention.
> But I was not going to change behaviour only to stop polluting
> logs with messages like above.

Your solution requires that someone has modified their definition to
include that startupPolicy='optional' attribute. I can be persuaded that
this is desirable; however, it would be nice to get Peter's opinion on
this especially since he knows the code and "rules" for backing stores
better than I do. IOW: From a storage backing chain perspective does it
make sense to use that.

>>> filter at least for cases of local files. Fixing other cases would
>>> require passing flag down the stack to .backendInit of storage
>>> which is ugly.
>> Yeah, I remember chasing down into backendInit (virStorageFileInitAs)
>> from qemuDomainStorageOpenStat for
>>    virStorageFileBackendForType:145 : internal error: missing storage
>> backend for 'none' storage
>>    virStorageFileBackendForType:141 : internal error: missing storage
>> backend for network files using iscsi protocol
>> where the 'none' comes from a volume using a storage pool of iSCSI
>> disks. I chased for a bit, but yes it got messy fast.
>>> Stats for active domain are fine because we either drop disks
>>> with unavailable sources or clean source which is handled
>>> by virStorageSourceIsEmpty in qemuDomainGetStatsOneBlockFallback.
>>> We have these logs for successful stats since 25aa7035d which
>>> in turn fixes 596a13713 which added substantial stats for offline
>>> disks.
>>> Signed-off-by: Nikolay Shirokovskiy <nshirokovskiy virtuozzo com>
>>> ---
>>>  src/qemu/qemu_driver.c  | 5 +++++
>>>  src/qemu/qemu_process.c | 9 +++++++++
>>>  src/qemu/qemu_process.h | 2 ++
>>>  3 files changed, 16 insertions(+)
>>> diff --git a/src/qemu/qemu_driver.c b/src/qemu/qemu_driver.c
>>> index a52e249..72e4cfe 100644
>>> --- a/src/qemu/qemu_driver.c
>>> +++ b/src/qemu/qemu_driver.c
>>> @@ -20290,6 +20290,11 @@ qemuDomainGetStatsBlockExportDisk(virDomainDiskDefPtr disk,
>>>      const char *backendstoragealias;
>>>      int ret = -1;
>>> +    if (!virDomainObjIsActive(dom) &&
>>> +        qemuProcessMissingLocalOptionalDisk(disk))
>>> +        return qemuDomainGetStatsBlockExportHeader(disk, disk->src, *recordnr,
>>> +                                                   records, nrecords);
>>> +
>> From my quick read this part would seem reasonable since the *Frontend
>> and *Backend wouldn't have valid data and *GetStatsOneBlock is fetching
>> source sizing data which for an empty source would be unobtainable.
>>>      for (n = disk->src; virStorageSourceIsBacking(n); n = n->backingStore) {
>>>          if (blockdev) {
>>>              frontendalias = QEMU_DOMAIN_DISK_PRIVATE(disk)->qomName;
>>> diff --git a/src/qemu/qemu_process.c b/src/qemu/qemu_process.c
>>> index 9cf9718..802274e 100644
>>> --- a/src/qemu/qemu_process.c
>>> +++ b/src/qemu/qemu_process.c
>>> @@ -6070,6 +6070,15 @@ qemuProcessPrepareSEVGuestInput(virDomainObjPtr vm)
>>>  }
>>> +bool
>>> +qemuProcessMissingLocalOptionalDisk(virDomainDiskDefPtr disk)
>> Curious why you chose qemu_process and not qemu_domain or even
>> virstoragefile?  This has nothing to do with whether the qemu process is
>> running or not.
> Yeah, we have nothing qemu specific here. Just not sure is this useful
> for other purpuses or not.

Since your chosen filter is "domain startup policy", I think qemu_domain
is where it belongs.

>>> +{
>>> +    return disk->startupPolicy == VIR_DOMAIN_STARTUP_POLICY_OPTIONAL &&
>> While I agree in principle about optional disks; however, since
>> startupPolicy is optional it would seem this particular check is chasing
>> a very specific condition. Would it be reasonable to use a flag instead?
>> Something passed on the command line to essentially say to only collect
>> the name/format the name of the local empty sources.
> Not sure I understand you. This patch does not change anything in respect
> to collected stats so I can not see why we need extra flags.

To be clear, I'm not advocating some new domain attribute/flag here.

The flag idea is something provided to virConnectGetAllDomainStats (or
of course virDomainListGetStats) rather than using startupPolicy. IOW,

Just "odd" that using the startupPolicy flag to avoid stats collection
for inactive guests and in the next patch for backing chains.  Perhaps
something worth adding to the startupPolicy description if this is the
option taken.
>> That way we're not reusing something that has other uses. Although I'm
>> open to other opinions.
>>> +           virStorageSourceIsLocalStorage(disk->src) && disk->src->path &&
>> Use virStorageSourceIsEmpty instead.
> But this is a bit different, say I use !virStorageSourceIsEmpty(src) instead of the line
> above, then I can not sure disk->src->path even makes sense in the below check.

Ah, right the !virStorageSourceIsLocalStorage && src->path would pass...
 Cannot mistake virStorageSourceIsEmpty for (non existent)

>>> +           !virFileExists(disk->src->path);
>> And while I believe I understand why you chose this, it would seem to me
>> that it might be useful to know if an optional local disk had a path
>> disappear.  IOW: If all the other conditions are met, I'd like to know
>> that the source path is missing. That might be a good thing to know if
>> I'm getting stats for a domain. Maybe that's just me.
> Yeah, its arguable. But as getting stats is periodic with rather high
> frequency seeing such log error over and over again in logs can be
> tedious.

Playing devil's advocate for a moment - what if someone didn't know the
$path to that backing store was missing? By at least logging somewhere
(but not fatal), the awareness can be made. By filtering the message if
the unrelated in name startupPolicy was set to optional, this knowledge
could be missed.

The concept of filtering is fine to me - the concern is really the usage
of the essentially unrelated startupPolicy to do so is what gives me
pause for concern. If there are others that are OK with this, it's not a
deal breaker for me.


>> BTW: I fixed a bug in the domstats path recently where the "last error"
>> was lost (see commit e1fc7ec08).  Although I don't think that's what
>> you're chasing here.
> Nope, it's different case.
> Nikolay
>> John
>>> +}
>>> +
>>> +
>>>  static int
>>>  qemuProcessPrepareHostStorage(virQEMUDriverPtr driver,
>>>                                virDomainObjPtr vm,
>>> diff --git a/src/qemu/qemu_process.h b/src/qemu/qemu_process.h
>>> index 2037467..52d396d 100644
>>> --- a/src/qemu/qemu_process.h
>>> +++ b/src/qemu/qemu_process.h
>>> @@ -214,4 +214,6 @@ int qemuProcessStartManagedPRDaemon(virDomainObjPtr vm);
>>>  void qemuProcessKillManagedPRDaemon(virDomainObjPtr vm);
>>> +bool qemuProcessMissingLocalOptionalDisk(virDomainDiskDefPtr disk);
>>> +
>>>  #endif /* __QEMU_PROCESS_H__ */

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