external zfs disk keeps writing

[Ronald Klop]

Hello,

I have a disk with ZFS which always writes something every  
'vfs.zfs.txg.timeout' seconds. By default that is 5 seconds. If I make it  
30 I see writing in gstat every 30 seconds.
I only do a backup by zfs send | zfs recv hourly on this disk. So I don't  
understand why it still writes something every 5 seconds while I am not  
doing anything to this disk.

How can I debug what is being written every 5 seconds?


Some info:

I am running FreeBSD 10-STABLE. The disk is an external disk connected by  
USB.

ugen3.2: <Philips> at usbus3
umass0: <Philips SPE3030CC, class 0/0, rev 2.00/0.00, addr 2> on usbus3
umass0:  SCSI over Bulk-Only; quirks = 0x4100
umass0:4:0:-1: Attached to scbus4
da0 at umass-sim0 bus 0 scbus4 target 0 lun 0
da0: <Hitachi HDT725032VLAT80 V54O> Fixed Direct Access SCSI-0 device
da0: 40.000MB/s transfers
da0: 305245MB (625142448 512 byte sectors: 255H 63S/T 38913C)
da0: quirks=0x2<NO_6_BYTE>

Occasionally I have a message like this, but it does not seem to hurt  
anything, so it is probably unrelated.

# bzgrep da0 /var/log/messages*
/var/log/messages.0.bz2:Sep  2 16:49:45 sjakie kernel:  
(da0:umass-sim0:0:0:0): WRITE(10). CDB: 2a 00 00 00 03 66 00 00 02 00
/var/log/messages.0.bz2:Sep  2 16:49:45 sjakie kernel:  
(da0:umass-sim0:0:0:0): CAM status: CCB request completed with an error
/var/log/messages.0.bz2:Sep  2 16:49:45 sjakie kernel:  
(da0:umass-sim0:0:0:0): Retrying command

# zpool status extern
   pool: extern
  state: ONLINE
   scan: scrub repaired 454K in 4h36m with 0 errors on Sat Aug  9 08:53:27  
2014
config:

	NAME        STATE     READ WRITE CKSUM
	extern      ONLINE       0     0     0
	  da0       ONLINE       0     0     0

errors: No known data errors

I just created a zpool on da0 so there is no partition table.

Regards,
Ronald.