BUG #10734: PostgreSQL 9.3.4 shutdown forever in zfsonlinux 0.6.3-1 filesystem

The following bug has been logged on the website:

Bug reference: 10734
Logged by: digoal.zhou
Email address: digoal(at)126(dot)com
PostgreSQL version: 9.3.4
Operating system: CentOS 6.5 x64
Description:

I have a machine DELL R720xd, and 1 ssd and 12 4TB SATA disks.
OS: CentOS 6.5 x64
ZFS: 0.6.3 installed by yum.
PostgreSQL 9.3.4.
zpool desc (12 SATA disks strip as vdev), (1 ssd partition as SLOG 2GB, and
another partition as L2ARC 800GB)
zpool ashift use 12. (4KB).
zfs nondefault parameters :
zp1/data01 recordsize 128K local
zp1/data01 mountpoint /data01 local
zp1/data01 checksum off local
zp1/data01 compression lz4 local
zp1/data01 primarycache metadata local

I use this dataset as $PGDATA.
and insert 500000000 row test data, use update by pkey and pgbench test it.
after all , checkpoint, vacuum freeze the table.
and then shutdown -m fast.
but shutdown very slow, about 10 mins.
and strace checkpoint_pid, I see some lseek and write system calls.
why?
it's a postgresql bug?