(Following up on @thcipriani's suggestion in our meeting to rename this to group_size, but with some explanation of how we might implement this internally.)

I think Scap supports this to some extent already via server_groups. All stages are executed over each group in a serial fashion, so if you wanted to deploy fully to each of the 10 targets in the given example, you would define one group per target host.

server_groups: one, two, three, ...
one_dsh_targets: /etc/dsh/group/one
two_dsh_targets: /etc/dsh/group/two
...

We could make this easier by supporting a per-group parameter that internally splits the group by a given size, resulting in the behavior you're describing but without the extra steps of manually defining separate groups.

server_groups: canary, main
canary_group_size: 1

In this example, if there were 10 canary hosts, each one would be represented internally by a single group (canary-1, canary-2, canary-3, etc.) and all stages of the deployment would be executed over each host in a serial fashion.

In T142990#2554038, @dduvall wrote:

We could make this easier by supporting a per-group parameter that internally splits the group by a given size, resulting in the behavior you're describing but without the extra steps of manually defining separate groups.

server_groups: canary, main
canary_group_size: 1

+1, yes, please, let's introduce (canary_)?group_size. There are 40+ Parsoid and 25+ RB hosts, so defining one-by-one groups manually is pretty cumbersome and error-prone.