Taking a page from something I learned from MySQL replication tricks, have you thought about trying a master -> inner slaves -> slaves out peers? Where there's one inner slave per datacenter/region.
I know atleast with 1.2.6 a slave can be a slave of a slave but I never measured the latency of a write at master, through the inner peers, out to the fan out slaves. Admittedly a more complicated topology but it would circumvent the stampede's against a master instance and also makes it easier to spin up larger numbers of slaves without wiping out the entire platform.
I know atleast with 1.2.6 a slave can be a slave of a slave but I never measured the latency of a write at master, through the inner peers, out to the fan out slaves. Admittedly a more complicated topology but it would circumvent the stampede's against a master instance and also makes it easier to spin up larger numbers of slaves without wiping out the entire platform.