Hi, Mike,

Apologies, I emailed this to cf-dev a few days ago, but it seems not to
have gone through. Anyway, thanks for asking about the different
configuration values Diego exposes for disk and memory. Yes, you can use
the 'diego.executor.memory_capacity_mb' and
'diego.executor.disk_capacity_mb' properties to specify overcommits in
absolute terms rather than the relative factors configurable on the DEAs.
The cell reps will advertise those values as their maximum memory and disk
capacity, and subtract memory and disk for allocated containers when
reporting their available capacity during auctions.

The 'btrfs_store_size_mb' property on garden-linux is more of a moving
target as garden-linux settles in on that filesystem as a backing store. As
of garden-linux-release 0.292.0, which diego-release 0.1412.0 and later
consume, that property accepts a '-1' value that allows it to grow up to
the full size of the available disk on the /var/vcap/data ephemeral disk
volume. The btrfs volume itself is sparse, so it will start at effectively
zero size and grow as needed to accommodate the container layers. Since
you're already monitoring disk usage on your VMs carefully and scaling out
when you hit certain limits, this might be a good option for you. This is
also effectively how the DEAs operate today, without an explicit limit on
the total amount of disk they allocate for containers.

If you do want more certainty in the maximum size that the garden-linux
btrfs volume will grow to, or if you're on a version of diego-release
earlier than 0.1412.0, you should set btrfs_store_size_mb to a positive
value, and garden-linux will create the volume to grow only up to that
size. One strategy to determine that value would be to use the maximum size
of the ephemeral disk, less the size of the BOSH-deployed packages (for the
executor, currently around 1.3 GB, including the untarred cflinuxfs2
rootfs), less the size allocated to the executor cache in the
'diego.executor.max_cache_size_in_bytes' property (which currently defaults
to 10GB).

Best,
Eric