Hello! I have a job that have multiple instances (>100) that'd I like to spread across the hosts in a cluster. Using a constraint such as "limit=host:1" doesn't work quite well, as I have more instances than nodes.
As a workaround I increased the limit value to something like ceil(instances/nodes). But now the problem happens if a bunch of nodes go down (think a whole rack dies) because the instances will not run until them are back, even though we may have spare capacity on the rest of the hosts that we'd like to use. In that scenario, the job availability may be affected because it's running with fewer instances than expected. On a smaller scale, the former approach would also apply if you want to spread tasks in racks or availability zones. I'd like to have one instance of a job per rack (failure domain) but in the case of it going down, the instance can be spawn on a different rack. I thought we could have a scheduling constraint to "spread" instances across a particular host attribute; instead of vetoing an offer right away we check where the other instances of a task are running, looking for a particular attribute of the host. We try to maximize the different values of a particular attribute (rack, hostname, etc) on the task instances assignment. what do you think? did something like this came up in the past? is it feasible? Mauricio
