I didn't have a /etc/modprobe.d/zfs.conf file
i created it
wrote options zfs zfs_arc_max = 8589934592
executed the command
update-initramfs -u
update-initramfs: Generating /boot/initrd.img-5.4.98-1-pve
Running hook script 'zz-pve-efiboot' ..
Re-executing '/etc/kernel/postinst.d/zz-pve-efiboot' in new private mount namespace ..
No / etc / kernel / pve-efiboot-uuids found, skipping ESP sync.
Nothing changed
You need to reboot for applying the changes this way.
For live changes you can write the module parameter directly as root user: echo "8589934592" > /sys/module/zfs/parameters/zfs_arc_max
(this is only for the current boot)