Hi Guys
Happy new year!
I would like to have two volumes (say /dev/data and /dev/binlog) which are used
by the same application to be synchronized from my primary node to the other
node (bin logs and data dir of MySQL). In DRBD docs it says that resource is a
replication group containing multiple volumes and “DRBD ensures write fidelity
across all volumes in the resource.”.
MySQL writes data to bin log and data log volumes and data dir contains
snapshot while binlog contains dirty segments. There is a relation between the
two - binlogs periodically merged back to data volume and purged, so to recover
on the other side data volume must contain recent enough snapshot such that
binlogs can be applied on top of it.
Following that I need two volumes to maintain write causality and be replicated
strictly together – i.e. avoid situation when one volume lags or doesn’t
replicate while the other one does, otherwise when I failover on the other side
I will observe inconsistent picture.
I put the following configuration to DRBD:
resource mysql_resource {
startup {
wfc-timeout 1;
}
disk {
no-disk-flushes;
no-md-flushes;
fencing resource-and-stonith;
on-io-error call-local-io-error;
disk-timeout 0;
}
volume 0 {
device /dev/drbd1;
disk /dev/data;
meta-disk internal;
}
volume 1 {
device /dev/drbd2;
disk /dev/binlog;
meta-disk internal;
}
on ip-10-15-0-135 {
address 10.15.0.135:1120;
}
on peer-host {
address 10.15.1.61:1120;
}
}
Now two questions:
Can I achieve such consistency guarantees by using DRBD resource with two
volumes?
If yes and a resource is a single logical replication unit (in a sense of
described guarantees), why are GIs different per volume? Can I have situations
when only one volume is disconnected or needs to resync? Can I have situations
when one volume is Priomary/Secondary while the other is Secondary/Primary?
More generally, what would be the motivation to have volumes grouped into
resources if synchronization still happens on volume level? (Or maybe I am
confused since documentation does refer to resources being unit of
synchronization (like sync bitmask is per reaource), but get/set -gi accepts
volume)
Anton Polyakov_______________________________________________
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