[libvirt] [RFC PATCH 2/2] lib: Add note that bulk stats API queries may overrun RPC buffers

[Martin Kletzander]

On Tue, May 23, 2017 at 09:32:03AM +0100, Daniel P. Berrange wrote:
>On Mon, May 22, 2017 at 06:00:13PM +0200, Peter Krempa wrote:
>> Hint that the users should limit the number of VMs queried in the bulk
>> stats API.
>> ---
>>  src/libvirt-domain.c | 7 +++++++
>>  1 file changed, 7 insertions(+)
>>
>> diff --git a/src/libvirt-domain.c b/src/libvirt-domain.c
>> index 310b91b37..b01f2705c 100644
>> --- a/src/libvirt-domain.c
>> +++ b/src/libvirt-domain.c
>> @@ -11315,6 +11315,10 @@ virConnectGetDomainCapabilities(virConnectPtr conn,
>>   * VIR_CONNECT_GET_ALL_DOMAINS_STATS_SHUTOFF and/or
>>   * VIR_CONNECT_GET_ALL_DOMAINS_STATS_OTHER for all other states.
>>   *
>> + * Note that this API is prone to exceeding maximum RPC message size on hosts
>> + * with lots of VMs so it's suggested to use virDomainListGetStats with a
>> + * reasonable list of VMs as the argument.
>> + *
>>   * Returns the count of returned statistics structures on success, -1 on error.
>>   * The requested data are returned in the @retStats parameter. The returned
>>   * array should be freed by the caller. See virDomainStatsRecordListFree.
>> @@ -11386,6 +11390,9 @@ virConnectGetAllDomainStats(virConnectPtr conn,
>>   * Note that any of the domain list filtering flags in @flags may be rejected
>>   * by this function.
>>   *
>> + * Note that this API is prone to exceeding maximum RPC message size on hosts
>> + * with lots of VMs so it's suggested to limit the number of VMs queried.
>
>With the way this is worded, applications have no guidance on what is a
>sensible max number of VMs they can safely use, so I don't think it is
>particularly useful, except as a way to say "we told you so" when apps
>report a bug.
>
>I'd be inclined to explicitly put a limit of 100 VMs[1] in the API, and
>document that hard limit, so apps immediately know to write their code
>to chunk in 100 VM blocks.
>

Or we can give a hint on what the limit is and let users figure out the
sensible number.  It is not only based on the number of VMs.  I believe
you can hit the limit with one or two VMs if you have lot of devices
that we report statistics for.  Limiting the number of VMs to a
particular number would not help as much in this case.  But we can
combine both approaches.

>Regards,
>Daniel
>
>[1] 100 is entirely arbitrary for this email - we'd actually pick a number
>    based on what we reasonably expect to fit in the RPC message size.
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