If a PUT request specifies storage classes, write to a volume that offers all specified classes. Otherwise, write to a volume that offers the "default" class.

X-Keep-Storage-Class: foo, bar

If there is no volume that offers all of the specified classes, return an error that causes the client not to retry the same request at the same server.

When checking whether data already exists, only check volumes that satisfy all of the desired classes.

source:services/keepstore/handlers.go#L744

Keep storage classes