Spare boxes

A spare is a box made ahead and joined to BB as a machine, which belongs to no thread. When a thread asks for a box of its kind, at the kind’s default size, it takes a spare and has it at once, rather than waiting while a box starts and joins BB.

Keep spares for a kind

In BB Box’s settings, go to Kinds and select the kind’s row.
Under Spare boxes, set Spares to keep to how many to keep ready, then select Save.

The row then says how many spares the kind keeps. BB Box makes them in the background, and makes another when a thread takes one.

A project’s image keeps its own spare too: one on each backend and kind it covers, while the image is in use. A box of the standard image does not take it.

Spares that wait stopped

On Proxmox’s VMs, android boxes among them, and Tart’s macOS and Windows VMs, a spare waits stopped once it is a BB machine, so it holds no memory.

Its machine’s name in BB says so, for example ‘calm-sparrow (spare, stopped)’.
BB’s machine list shows it offline meanwhile. This is expected.
A thread that takes it waits a few seconds while it resumes and its machine connects again.
Each such spare also takes the place of one of the kind’s warm boxes, so the host holds no more than before.

Spares of device boxes

ios and android boxes keep no spares unless you set a number. Their spares are off by default because each one costs disk while it waits. Set Spares to keep on the kind’s page to turn them on.

An android spare waits as a hibernated Proxmox VM with its device booted. It holds no memory. Its saved memory takes about as much disk as the box’s memory: 16.8 GiB for a box of 16 GiB, measured.
An ios spare on Tart waits as a suspended macOS VM with its device booted. It takes none of the Mac’s two VM slots. Its saved memory takes less disk than the box’s memory: 5.2 GiB for a box of 16 GiB, measured.
A spare stops to wait only once its device has booted. A spare whose device does not boot within 10 minutes is released, and BB Box makes no other spare of that kind for 30 minutes.
A spare holds the kind’s default device. A thread that asks for another device takes the spare and the device switches. If the switch fails, the box keeps the spare’s device, and box_acquire says why.
A Tart spare is replaced once it has waited longer than Tart: hours a macOS warm VM or spare is kept, 6 by default, as a saved state that old may not restore. If a spare’s saved state does not restore, its VM boots afresh from its disk and box_acquire says so.

The doctor’s Spares section says, for each device kind, whether spares are off, how many are kept and ready, and what each costs. It warns when a kind has fewer spares than its number, and why.

What a claimed spare says

In New box, a provider row says ‘ready now’ when a spare of the size you chose is waiting there, or ‘ready in seconds’ when that spare waits stopped and resumes first. See making a box.

Once a thread has taken it, the box’s row says ‘BB machine in N s’, as a new box does. For a spare that waited stopped, N is how long its machine took to connect again after it resumed.

Spares stay current

BB Box replaces a spare it no longer wants. It does this when:

the kind’s image or template was rebuilt since the spare was made, as when standard images are rebuilt, so a thread never takes a box of the old build
the kind’s default size changed
you lower the number of spares

A spare of a project’s image is replaced in the same way when the image’s build changes.