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I sync my new disk, output of drbdtop:

Resource: myres: (Overall danger score: 14)
Local Disc(Primary):
volume 0 (/dev/drbd0): UpToDate(normal disk state)
volume 1 (/dev/drbd1): UpToDate(normal disk state)
volume 2 (/dev/drbd2): Inconsistent(data is not accessible or usable until resync is complete)

Connection to zfs.user.osdc2(Secondary): Connected(connected to zfs.user.osdc2)
volume 0: 
UpToDate(normal disk state)
volume 1:
UpToDate(normal disk state)
volume 2:
Replication:SyncTarget(local volume is being synchronized with data from zfs.user.osdc2) 95.6% remaining UpToDate(normal disk state) 

But there is no output in cat /proc/drbd about my resource. Only:

version: 9.0.9-1 (api:2/proto:86-112)
GIT-hash: f7b979e7af01813e031aac579140237640c94569 build by mockbuild@, 2017-09-14 17:45:45
Transports (api:16): tcp (9.0.9-1)

Why there is no output? How should I resolve this issue?

The /proc/drbd file has been dropped in the DRBD 9.x releases. You should instead use drbdadm status to get the current state of the DRBD resources.

If you really insist on the old /proc/drbd output you can find this within the sysfs now. Note that this is per connection only. It is located at:

/sys/kernel/debug/drbd/resources/$resource_name/connections/$hostname/0/proc_drbd