What append when arc full

[Ben RUBSON]

On 14 Jun 2018 07:43, Albert Shih wrote:

> I try last night, if I put
>
>   vfs.zfs.arc_max=96G
>
> everything work fine (the sending). So I think that's the point.
>
> But my concern are what append if I shrink a little more the arc_max, let's
> say 64G knowing I got < 30To of data. Does I'm going to loose lot of
> performance ?

The least recently used buffers of (meta)data (the colder ones) will be  
evicted to make room for the new needed (meta)data buffers.
ZFS has a "constantly" background job running to keep ARC size under the  
limit.
Performance will of course depend on the amount of hot (meta)data you play  
with.
Perhaps you could benefit from setting a L2ARC with your 2 SSDs.
Evicted buffers would then be stored there for faster access when needed.

>> If you want to keep some ram free for your processes, then you can set
>> vfs.zfs.arc_max to limit the ram used by ARC. On recent system versions
>> you can change it dynamically using sysctl or you can set it in
>
> "dynamically" .... I though it's only can set at boot time

I also have it in /etc/sysctl.conf on 11.0 systems, I think it was also the  
case with 10.3.

Ben