Varnish Shared memory Log can put a major strain on the disk (I/O write operations). The solution is to store it on the virtual memory filesystem.
Define tmpfs virtual memory filesystem on /var/lib/varnish directory.
$ echo "tmpfs /var/lib/varnish tmpfs rw,size=1G 0 0" | sudo tee -a /etc/fstab
Stop varnish service.
$ sudo systemctl stop varnish
Mount tmpfs virtual memory filesystem.
$ sudo mount /var/lib/varnish
Start varnish service.
$ sudo systemctl start varnish
The tmpfs virtual memory filesystem will get populated as expected.
$ df -h /var/lib/varnish/
Filesystem Size Used Avail Use% Mounted on tmpfs 1.0G 81M 944M 8% /var/lib/varnish