After a week of so of running the server as a backup to my main NAS, I had a quick look at the logs and found the syslog being swamped with these messages, here are some samples off the top of the syslog:
May 31 08:22:48 raspberrypi collectd[19041]: rrdcached plugin: rrdc_update (/var/lib/rrdcached/db/localhost/interface-eth0/if_packets.rrd, [1464679368:7210783:1749906], 1) failed with status -1.
May 31 08:22:48 raspberrypi collectd[19041]: Filter subsystem: Built-in target `write': Dispatching value to all write plugins failed with status -1.
May 31 08:22:48 raspberrypi collectd[19041]: rrdcached plugin: rrdc_update (/var/lib/rrdcached/db/localhost/interface-eth0/if_errors.rrd, [1464679368:0:0], 1) failed with status -1.
May 31 08:22:48 raspberrypi collectd[19041]: Filter subsystem: Built-in target `write': Dispatching value to all write plugins failed with status -1.
May 31 08:22:48 raspberrypi collectd[19041]: rrdcached plugin: rrdc_update (/var/lib/rrdcached/db/localhost/df-media-fa36508a-b3c4-4499-b30a-711dd5994225/df_complex-reserved.rrd, [1464679368:211551232.000000], 1) failed with status -1.
May 31 08:22:48 raspberrypi collectd[19041]: Filter subsystem: Built-in target `write': Dispatching value to all write plugins failed with status -1.
May 31 08:22:48 raspberrypi collectd[19041]: rrdcached plugin: rrdc_update (/var/lib/rrdcached/db/localhost/df-media-457474be-d970-4f9d-9c0b-84a235546ae0/df_complex-reserved.rrd, [1464679368:16777216.000000], 1) failed with status -1.
May 31 08:22:48 raspberrypi collectd[19041]: Filter subsystem: Built-in target `write': Dispatching value to all write plugins failed with status -1.
May 31 08:22:48 raspberrypi collectd[19041]: rrdcached plugin: rrdc_update (/var/lib/rrdcached/db/localhost/rrdcached/queue_length.rrd, [1464679368:0.000000], 1) failed with status -1.
May 31 08:22:48 raspberrypi collectd[19041]: Filter subsystem: Built-in target `write': Dispatching value to all write plugins failed with status -1.
May 31 08:22:48 raspberrypi collectd[19041]: rrdcached plugin: rrdc_update (/var/lib/rrdcached/db/localhost/rrdcached/operations-receive-update.rrd, [1464679368:2340607], 1) failed with status -1.
May 31 08:22:48 raspberrypi collectd[19041]: Filter subsystem: Built-in target `write': Dispatching value to all write plugins failed with status -1.
May 31 08:22:48 raspberrypi collectd[19041]: rrdcached plugin: rrdc_update (/var/lib/rrdcached/db/localhost/rrdcached/operations-receive-flush.rrd, [1464679368:47300], 1) failed with status -1.
May 31 08:22:48 raspberrypi collectd[19041]: Filter subsystem: Built-in target `write': Dispatching value to all write plugins failed with status -1.
May 31 08:22:48 raspberrypi collectd[19041]: rrdcached plugin: rrdc_update (/var/lib/rrdcached/db/localhost/rrdcached/operations-write-updates.rrd, [1464679368:18], 1) failed with status -1.
May 31 08:22:48 raspberrypi collectd[19041]: Filter subsystem: Built-in target `write': Dispatching value to all write plugins failed with status -1.
May 31 08:22:48 raspberrypi collectd[19041]: rrdcached plugin: rrdc_update (/var/lib/rrdcached/db/localhost/rrdcached/operations-write-data_sets.rrd, [1464679368:18], 1) failed with status -1.
May 31 08:22:48 raspberrypi collectd[19041]: Filter subsystem: Built-in target `write': Dispatching value to all write plugins failed with status -1.
May 31 08:22:48 raspberrypi collectd[19041]: rrdcached plugin: rrdc_update (/var/lib/rrdcached/db/localhost/rrdcached/gauge-tree_nodes.rrd, [1464679368:55.000000], 1) failed with status -1.
May 31 08:22:48 raspberrypi collectd[19041]: Filter subsystem: Built-in target `write': Dispatching value to all write plugins failed with status -1.
May 31 08:22:48 raspberrypi collectd[19041]: rrdcached plugin: rrdc_update (/var/lib/rrdcached/db/localhost/rrdcached/gauge-tree_depth.rrd, [1464679368:7.000000], 1) failed with status -1.
My NAS consists of an RPi Model 2 with 2 x 1TB hard disks running as a RAID 1 (mirror) and everything appears to be working correctly, with my Synology NAS backing up everything to the RPi absolutely fine. Clearly there is a configuration error somewhere. Research of some really old threads suggests that it is a problem with connectd and to look at collectd.conf, but I don't know what I am looking for but these are current contents:
Hostname "localhost"
FQDNLookup true
LoadPlugin syslog
<Plugin syslog>
LogLevel info
</Plugin>
LoadPlugin rrdcached
<Plugin rrdcached>
DaemonAddress "unix:/var/run/rrdcached.sock"
DataDir "/var/lib/rrdcached/db/"
CreateFiles true
CollectStatistics true
</Plugin>
LoadPlugin unixsock
<Plugin unixsock>
SocketFile "/var/run/collectd.socket"
SocketGroup "root"
SocketPerms "0660"
</Plugin>
LoadPlugin cpu
LoadPlugin df
<Plugin df>
# MountPoint "/"
MountPoint "/media/fa36508a-b3c4-4499-b30a-711dd5994225"
MountPoint "/media/457474be-d970-4f9d-9c0b-84a235546ae0"
IgnoreSelected false
</Plugin>
LoadPlugin interface
<Plugin interface>
Interface "eth0"
IgnoreSelected false
</Plugin>
LoadPlugin load
LoadPlugin memory
Can anyone point me in the right directon?