sync timing

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ictblankenberge
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sync timing

Unread post by ictblankenberge » Thu Jun 28, 2018 9:48 am

Hi everyone,
First time poster here.

We've recently "dug" up 2 RS814RP+ and want to use them as file servers. I've come across the SHA and want to configure it so that if one server dies the other takes over.
Now since these 2 servers will be in different buildings in different parts of the city and we only have a 2GB connection between the two buildings we don't want them syncing all the time as to not overload the network. Is there a possibility that the data sync can be done at certain intervals?

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jjb2
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Re: sync timing

Unread post by jjb2 » Thu Jun 28, 2018 12:52 pm

Can't be done. Units need to be right next to each other. Ok, that is a tad inaccurate - it can be done but you don't want to go down that route as it involves a dedicated strand of fiber/cat5 between the buildings, no switches, no latency, etc.
So many Synology systems for so many years starting at DSM V1.x.

ictblankenberge
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Re: sync timing

Unread post by ictblankenberge » Tue Jul 03, 2018 10:50 am

Wow, that's like highly unpractical, so if you want to CYA against a burning building and place your server in another building you can't do it unless you have a dedicated line between the two?

So you can't say that the sync only runs once every let's say 6h, it has to happen real time?

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Re: sync timing

Unread post by parnassus » Tue Jul 03, 2018 6:52 pm

Synology HA (Active/Passive) works - as other similar HA solutions - that way because Heartbeat link(s) need to be back-to-back directly linking HA's member Synology NAS units (no Switch in between, no Layer 3 in between)...it's a deployment one does within a building not between geographical sites (at least when they are not connected directly as when you have Short Range/Long Range Fiber Optics links or just copper links).

Another path would be replicating snapshots off-site (on the remote site) but that implementation can't be classified into an High Availability solution, it's just a (type of) replication solution.

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Re: sync timing

Unread post by ictblankenberge » Thu Jul 12, 2018 2:31 pm

parnassus wrote:
Tue Jul 03, 2018 6:52 pm
Synology HA (Active/Passive) works - as other similar HA solutions - that way because Heartbeat link(s) need to be back-to-back directly linking HA's member Synology NAS units (no Switch in between, no Layer 3 in between)...it's a deployment one does within a building not between geographical sites (at least when they are not connected directly as when you have Short Range/Long Range Fiber Optics links or just copper links).

Another path would be replicating snapshots off-site (on the remote site) but that implementation can't be classified into an High Availability solution, it's just a (type of) replication solution.
The buildings are connected via microwave antennas and rely on Line of Sight so techically they are all connected in the same network.
But we tried to setup the SHA it said that if it was connected to a switch it needed it's own VLAN to setup the Heartbeat link. SO technically the thing you are saying isn't possible is possible. I just wanted to now that if the sync HAS to happen continously or if you can say sync once every 6h.
Also it seems rather useless to setup a SHA solution that shouldn't be able to be located on another geographical site. So if the building that has the first device burns down, has a power failure, gets flooded, ... the other one would have the same problem and your SHA solution would offer 0 help.

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Re: sync timing

Unread post by parnassus » Fri Jul 13, 2018 11:50 pm

Synchronization between Active node and Passive node is continuous...is synchronuos (and not delayable at user convenience)...at least this is the default operating mode internally configured on DRBD.

Synology HA works this way.

As said nobody will stop you to deploy a Synology HA on one (main) site and then - exactly because you can tolerate a 6 hours synchronization time window as you wrote - to perform snapshots every 6 hours or so against your second (near/far) remote site...or try to deploy (since hacking DRBD on your own Synology HA solution wouldn't be so adviseable) your fully customized DRBD HA using two Linux servers.

Synology recommends back-to-back - so direct - Heartbeat (fast/very fast) single link or aggregated links (for resiliency)...that's the way Synology HA should be done to avoid any type of communication issue between Active node and Passive node...it's also a way to say that Synology HA, and it is generally valid for other similar HA implementations, needs very low latency values on the Heartbeat link...values that can be easily obtained on 1Gbps or 10Gbps direct (single or aggregated) links.

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Re: sync timing

Unread post by jjb2 » Thu Jul 26, 2018 10:57 am

Actually you can do this - if you read some of my old HA posts I explain it to someone who asked a year or two ago about it. I don't, however, necessarily recommend it in most cases. I will say that we have a HA setup with one unit over 1000 miles from the first but the locations they are used in have less than a 1ms ping time between them which is why it could be done. Good luck.
So many Synology systems for so many years starting at DSM V1.x.

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Re: sync timing

Unread post by parnassus » Thu Jul 26, 2018 7:31 pm

1 ms (millisecond) for a link long 1000 miles? it sounds a little bit optimistic (it's a LAN performance over a WAN links with some devices in between)...that's considering just about 6 ms per each 1000 miles as an absolute physical transmission limit (which means a grandtotal of 12 ms round-trip time).

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Re: sync timing

Unread post by jjb2 » Wed Aug 01, 2018 11:37 am

Undersea fiber has almost no lag so its possible to reduce ping times to almost zero, however, not economical in any sense and it our case it was just a test that turned out more effort than it was worth. Like I mentioned, not recommended but anything is possible it seems.
So many Synology systems for so many years starting at DSM V1.x.

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