[libvirt] [PATCH] qemu: add entry for balloon stat stat-disk-caches

[Michal Privoznik]

On 06/05/2018 03:08 PM, Erik Skultety wrote:
> On Tue, Jun 05, 2018 at 01:41:02PM +0200, Tomáš Golembiovský wrote:
>> Signed-off-by: Tomáš Golembiovský <tgolembi at redhat.com>
>> ---
>>  include/libvirt/libvirt-domain.h | 9 ++++++++-
>>  src/qemu/qemu_monitor_json.c     | 2 ++
>>  2 files changed, 10 insertions(+), 1 deletion(-)
>>
>> diff --git a/include/libvirt/libvirt-domain.h b/include/libvirt/libvirt-domain.h
>> index da773b76cb..b96c018a90 100644
>> --- a/include/libvirt/libvirt-domain.h
>> +++ b/include/libvirt/libvirt-domain.h
>> @@ -628,11 +628,18 @@ typedef enum {
>>      /* Timestamp of the last update of statistics, in seconds. */
>>      VIR_DOMAIN_MEMORY_STAT_LAST_UPDATE     = 9,
>>
>> +    /*
>> +     * The amount of memory, in bytes, that can be quickly reclaimed without
>> +     * additional I/O. Typically these pages are used for caching files from
>> +     * disk.
>> +     */
>> +    VIR_DOMAIN_MEMORY_STAT_DISK_CACHES     = 10,
>> +
>>      /*
>>       * The number of statistics supported by this version of the interface.
>>       * To add new statistics, add them to the enum and increase this value.
>>       */
>> -    VIR_DOMAIN_MEMORY_STAT_NR              = 10,
>> +    VIR_DOMAIN_MEMORY_STAT_NR              = 11,
> 
> this is a public header, this must never change, otherwise you break backwards
> compatibility...

Not true. This is meant to work roughly like this:

  virDomainMemoryStatStruct stats[VIR_DOMAIN_MEMORY_STAT_NR];

  nr_stats = virDomainMemoryStats(dom, stats, VIR_DOMAIN_MEMORY_STAT_NR, 0);

And depending what version you are compiled with you will get different
number of results. Important to say that to maintain backward
compatibility we have a rule that says _NR can only grow and never
shrink. Tomas' patch is actually correct (in this aspect).
View examples:

200a40f94ec9427eb7187d9d5396ad3a3f2925c8
65bf044686c7502ba16f1bee5763fd3e448994fd


Michal