Resource Monitor under-estimates ARC size

[taoyouh12138]

As shown below, the memory usage is shown as only 71.5% in Resource Monitor, while swap is used heavily and the cache is really low. This looked weird as I believe in a general Linux system, after running steadily for long enough time and doing enough I/O, ARC and cache usage should go high and take almost all available RAM, but the Resource Manager showed a lot of “Free” memory not being utilized by anything.

I SSHed to the machine and used free and cat /proc/lpl/kstat/zfs/arcstats to get some numbers. It turns out that the actual used memory from free is much higher than shown in Resource Manager, and the ZFS ARC size shown is also different from two sources.

And I found 2 coincidences:

1. The utilization in Resource Manager (11.03GB) is very close to used - buffers/cache - arc evictable from SSHed values: (13758 - 2590921728 / 1024 / 1024) / 1024 ~= 11.02
2. The ZFS ARC shown in Resource Manager (3.63GB) is very close to size - evictable from SSH: (6492646377 - 2590921728) / 1024 / 1024 / 1024 = 3.63

So, is QNAP deliberately hiding the “evictable” part of ARC usage in the Resource Manager? This is quite misleading. Even it’s evictable it isn’t as available as the “Free” memory: the system does not reclaim it very actively for apps or anything. And we also can’t see the real ARC usage from Resource Manager.

I hope QNAP could show the “evictable” part of ARC in Resource Manager.

QuTS Hero version: h5.2.8.3350

$ free -m
             total       used       free     shared    buffers     cached
Mem:         15787      14212       1575        114          3        423
-/+ buffers/cache:      13785       2002
Swap:        62242       4901      57341

$ cat /proc/lpl/kstat/zfs/arcstats
...
evictable                       4    2590921728
...
c_max                           4    9270338840
size                            4    6492646377

[Becker2020]

Hello,

there is the same post in the german sektion

QuTShero, der Ressourcenmonitor zeigt falsche RAM-Werte an

Die Anzeigen des Ressourcenmonitor unter QUTShero (h5.2.8.3359) zum ZFS-ARC und dem freien RAM sind falsch. QNAP hat bei hero die Werte für arc_max und arc_min fest hinterlegt. → arc_max ist ca. 80% des verbauten RAM → arc_min ist ca. 20% des verbauten RAM (bei 64 GB sind dies die 11,71 GB) → mit der Einstellung des nutzbaren ARC (40%, 50%,…) wird der tatsächlich nutzbare ARC geregelt. → 50% vom arc_max würden dann einem ARC von ca. 25 GB (50% von 80%) entsprechen. Diesen Wert sieht man abe…

[taoyouh12138]

Yes I think they’re the same issue

[Lucas]

Hi @taoyouh12138 ,

Thank you for reporting this issue and for sharing the detailed comparison between Resource Monitor, free, and /proc/lpl/kstat/zfs/arcstats.

We understand your concern that the ZFS ARC usage shown in Resource Monitor may not fully reflect the actual ARC usage, especially the evictable ARC portion, which could make the displayed free memory and ARC size look different from what is observed through SSH.

We will arrange for our team to review and analyze this behavior further. Your detailed calculation and observations are helpful for our investigation.

Thank you again for bringing this to our attention.