[SOLVED] Question about ram usage

[Lucas Rey]

Hello community,
I'm just checked the ram usage from GUI and OS, and I can see something not clear to me.

GUI shown a critical ram usage value:


While the OS shown different ram usage (htop):


So, is it a situation where I have to worry about? Which source should I believe?

Thank you
Lucas

 

[Dunuin]

Whats your output of free -hg? Top and similar programs often will show "available" RAM comparable to "free" RAM even if it is actually used.

 

[Lucas Rey]

Ah yes, you're right. I remembered now that htop is not a trusted source!!
So, it's time to think about RAM upgrade

# free -hg
               total        used        free      shared  buff/cache   available
Mem:           125Gi       119Gi       5.5Gi        69Mi       425Mi       4.8Gi
Swap:             0B          0B          0B

Thank you
Lucas

 

[Dunuin]

RAM at 90+% doesn't mean you have to be out of RAM. For example in case you re using ZFS then by default up to 50% of your RAM will be used by ZFS as its ARC for caching. ARC won't be listed as "buff/cache" but as normal "used" but ZFS can shrink it dynamically but slowly from for example 64GB down to 1GB if needed by processes.

 

[Lucas Rey]

RAM at 90+% doesn't mean you have to be out of RAM. For example in case you re using ZFS then by default up to 50% of your RAM will be used by ZFS as its ARC for caching. ARC won't be listed as "buff/cache" but as normal "used" but ZFS can shrink it dynamically but slowly from for example 64GB down to 1GB if needed by processes.

Aaaahh, you're right!! ZFS is used in local disk, where proxmox is installed (SSD mirror), and in several VMs like TrueNAS and PFSense.

        NAME                                       STATE     READ WRITE CKSUM
        rpool                                      ONLINE       0     0     0
          mirror-0                                 ONLINE       0     0     0
            ata-CT480BX500SSD1_2030E4089A27-part3  ONLINE       0     0     0
            ata-CT480BX500SSD1_2006E3E9834D-part3  ONLINE       0     0     0

But how can I determine is the used ram is allocated to ZFS or VMs? Or better, is there a way, via cli, to check how many ram are using the VMs?

Is this the ram used by ZFS?

ARC size (current):                                    73.5 %   46.0 GiB
        Target size (adaptive):                        73.5 %   46.0 GiB
        Min size (hard limit):                          6.2 %    3.9 GiB
        Max size (high water):                           16:1   62.6 GiB
        Most Frequently Used (MFU) cache size:         64.6 %   29.5 GiB
        Most Recently Used (MRU) cache size:           35.4 %   16.2 GiB
        Metadata cache size (hard limit):              75.0 %   47.0 GiB
        Metadata cache size (current):                  1.0 %  485.2 MiB
        Dnode cache size (hard limit):                 10.0 %    4.7 GiB
        Dnode cache size (current):                     0.7 %   34.0 MiB

Thank you
Lucas

 

[Dunuin]

Aaaahh, you're right!! ZFS is used in local disk, where proxmox is installed (SSD mirror), and in several VMs like TrueNAS and PFSense.

        NAME                                       STATE     READ WRITE CKSUM
        rpool                                      ONLINE       0     0     0
          mirror-0                                 ONLINE       0     0     0
            ata-CT480BX500SSD1_2030E4089A27-part3  ONLINE       0     0     0
            ata-CT480BX500SSD1_2006E3E9834D-part3  ONLINE       0     0     0

But how can I determine is the used ram is allocated to ZFS or VMs? Or better, is there a way, via cli, to check how many ram are using the VMs?

Is this the ram used by ZFS?

ARC size (current):                                    73.5 %   46.0 GiB
        Target size (adaptive):                        73.5 %   46.0 GiB
        Min size (hard limit):                          6.2 %    3.9 GiB
        Max size (high water):                           16:1   62.6 GiB
        Most Frequently Used (MFU) cache size:         64.6 %   29.5 GiB
        Most Recently Used (MRU) cache size:           35.4 %   16.2 GiB
        Metadata cache size (hard limit):              75.0 %   47.0 GiB
        Metadata cache size (current):                  1.0 %  485.2 MiB
        Dnode cache size (hard limit):                 10.0 %    4.7 GiB
        Dnode cache size (current):                     0.7 %   34.0 MiB

Thank you
Lucas

Jup, arc_summary is best for that. So right now ZFS is using 46 GiB for caching and that can change between 4GB and 64GB. So there is plenty of RAM left if you just use ZFS on your host for the two boot disks.
Rule of thumb for recommended ARC size is 2/4GB + 0.25-1 GB per 1 TB of raw storage when not using deduplication...it varies depending on who you ask.

You can also limit the ARC size as shown there: https://pve.proxmox.com/wiki/ZFS_on_Linux#sysadmin_zfs_limit_memory_usage

 

[Lucas Rey]

You can also limit the ARC size as shown there: https://pve.proxmox.com/wiki/ZFS_on_Linux#sysadmin_zfs_limit_memory_usage

I see thank you. What is not clear to me is if such 46 GiB, currently used by ZFS, can be automatically yield in case system needs more ram, without I'll messing with ZFS settings.

Lucas

 

[Dunuin]

It will be freed up automatically when needed, but it can't be freed up as fast as the normal linux cache. Most of the time it should be fine but sometimes when alot of memory is needed very fast it can happen that you still run out of space. If such a thing is happening (monitor syslog for OOM messages) it might be useful to manually limit the ARC size.

 

[Lucas Rey]

Thank you very much, I limited the ARC size to 10 GiB and this is the result


Good!