zfs_arc_max realtime

[spearox]

Dear Members,

I would like to ask about the /etc/modprobe.d/zfs.conf
Can I change the options zfs zfs_arc_max=X parameter realtime somehow? There are running VM's and I don't would like to restart.

Thanks.

 

[Alwin]

No, the options are loaded when the module is inserted into the kernel.

 

[efeu]

Ehm, you can w/o reboot by:

echo "your wanted size in byte" > /sys/module/zfs/parameters/zfs_arc_max

and a

echo 3 > /proc/sys/vm/drop_caches

 

[spearox]

Ehm, you can w/o reboot by:

echo "your wanted size in byte" > /sys/module/zfs/parameters/zfs_arc_max

and a

echo 3 > /proc/sys/vm/drop_caches

Thanks!

Can you tell me please what this do exactly? "echo 3 > /proc/sys/vm/drop_caches"

 

[efeu]

Well, if you want to decrease your ARC, you need to free the already allocated memory.

https://linux-mm.org/Drop_Caches

Running that drop_caches command _should_ free the reclaimable RAM from the ARC. There are some deficiencies in the ZFS on Linux memory management though, which could cause this *not* to happen

To set the max_size "more save" w/o a host reboot, you could just un-/ and reload zfs module, but forsure this needs to shutdown all processes which use ZFS (e.g. your VMs on ZFS pool)

 

[Alwin]

To free pagecache:
echo 1 > /proc/sys/vm/drop_caches
To free reclaimable slab objects (includes dentries and inodes):
echo 2 > /proc/sys/vm/drop_caches
To free slab objects and pagecache:
echo 3 > /proc/sys/vm/drop_caches

https://www.kernel.org/doc/Documentation/sysctl/vm.txt