Required # of Sync channels for each Client channels for HA

[logicmate]

My understanding for an HA setup was that for each client channel available there should be a Sync Channel between the HA Nodes, below are snippets from my older posts.

logicmate wrote:
When we install 5.8, if HA-Node-1 has 2x1GB NICs for iSCSI client and HA-Node-2 has 2x1GB NICs for iSCSI client that would be 4x1GB NICSs for iSCSI clients total (theoretically 4000mbs total bandwidth with MPIO). How many NICs should be used for the sync channel? would we need 4x1GB NICs per server for sync? or 2x1GB per server is enough?

Anatoly (staff):
Total bandwidth of Sync channel should be not less then total bandwidth of all the client connections.

So in theory (and the setup we have right now) is as follows

• HA-Node #1 Windows 2008 x64 R2 - iSCSI SAN
  • 4 x 1GB Sync channels
    2 x 1GB iSCSI/Client channels
  HA-Node #2 Windows 2008 x64 R2 - iSCSI SAN
  • 4 x 1GB Sync channels
    2 x 1GB iSCSI/Client channels

Now taking into the fact that the NICS are full-duplex (meaning they can simultaneously send & receive 1GB each way) why do we need 4 Sync channels? Can't we get away with only 2 Sync Channels per node?

This had bugged me for a while but I just accepted the fact, however looking at my network utilization I noticed the following (example):

• HA-Node #1
  • Sync Channel #1 20%
    Sync Channel #2 20%
    Sync Channel #3 20%
    Sync Channel #4 20%
    
    Client Channel #1 40%
    Client Channel #2 40%
  HA-Node #2
  • Sync Channel #1 20%
    Sync Channel #2 20%
    Sync Channel #3 20%
    Sync Channel #4 20%
    
    Client Channel #1 40%
    Client Channel #2 40%

Now my question is, which of the following is true?

1. Total bandwidth of Sync channel should be not less then total bandwidth of all the client connections per node
or
2. Total bandwidth of Sync channel should be not less then total bandwidth of all the client connections combined of all nodes


Thanks,
Ara

[@ziz (staff)]

Actually this cannot be used as a rule. All depends on the load on the HA partners, the disks performance and it differs from an environment to another.
In your case I suggest you to disable one of the sync channel links (or two of them) and look at the new rates you'll get.