Hi Guys

   I am once again having a weird issue... well weird to me anyway.

   We have a master,slave setup using mysql 5.7 and they are both
connected on the same network segment through the same switch.

  During the weekend a HUGE amount of processing was done on the master,
and thus resulted in the slave lagging and is currently on 213000
seconds behind master.

    While troubleshooting, I found the following :

     - MySQL io thread (relay log positions) are lagging far behind what
is currently on master server.
      - SQL thread processes as the io thread are writing the relay log
with no lag there.

       As a test , I disabled sql thread and let the io thread sync binlog
data to relay log, while monitoring network traffic.

     The network traffic would max out at 6mb/s and as it needs to sync
over 200Gb of binary logs, this seems to be taking too long.

      I then suspected a network issue, however when using SCP to copy
data from the master server to slave server I get speeds of up to
80Mb/s for those copies.

       When I start the sql thread again after about 2 gb of data have
been synced, then it catches up within a minute.

        So my question is this..

          are there any options to mysql that would be causing it to limit
the speed at which it sync binary logs.

          If not, how else can I speed up that process.

          FYI, the scp that was done was done to the same disks where the
relay logs are on in order to also confirm that it is not a disk
io issue, however that went more than good enough.

        So from what we can see the bottleneck is specifically related to
slow sync over the network and seems to be only related to mysql.

  Any ideas here would be appreciated.


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