HUGE slowdown when doing dpkg with ext4 over nbd

Hello,

We have noticed a HUGE slowdown when doing dpkg with ext4 over nbd (qemu-nbd as a server)

dpkg times with default mkfs & mount options is :

- ext4: 10 mn
- xfs : 30s.

Disabling extents : 
mkfs.ext4 -O ^extend
fixed the pb that is ext4 is as fast as xfs.

Any ideas ?
Renaud


Details :

xnbd-client  --blocksize 4096  qemu-nbd-ip 10000 /dev/nbd0
mkfs.ext4 /dev/nbd0
mount /dev/nbd0 /newroot

wget https://download.elastic.co/kibana/kibana/kibana-4.6.1-amd64.deb
time dpkg -i kibana-4.6.1-amd64.deb

kibana installs many small files (23209 entries) under /opt/kibana