Enum ReservationOptions 

pub enum ReservationOptions<'a> {
    New,
    Borrowed,
    Hold(&'a Reservation),
}

How metadata and allocator reservations should be handled for a transaction.

A transaction consumes space in two different ways:

1. The transaction will consume space in the journal and eventually in an LSM tree (the “metadata” cost of the transaction).
2. The transaction might also include additional allocations or deallocations. These may come directly from the global allocator, or they might come from a Reservation object, depending on the context. (See specific enum cases for details.)

The basic decision tree for which variant to use is as follows:

• Is there an existing reservation which the transaction should draw from?
  • YES: Use ReservationOptions::Hold
  • NO: Is the transaction allocating or deallocating extents for internal filesystem metadata (e.g. LSM tree compaction, journal/superblock growth, or graveyard tombstoning in the root/root-parent stores) or expected to be space-neutral or space-saving? - YES: Use ReservationOptions::Borrowed - NO: Use ReservationOptions::New

Variants

New

A new reservation is created to accommodate the metadata of a maximally-sized transaction, and the transaction’s metadata is paid for from this reservation. Allocations and deallocations within the transaction will go directly to the global allocator (and therefore might fail if there’s insufficient space).

This should be the default case for most transactions which permit failure when there’s no space left in the global allocator.

Borrowed

Metadata space is borrowed directly from the global reservation held by the ObjectManager.

For system stores (root_parent_store and root_store), extent allocations and deallocations in the transaction also draw from and return space to the global metadata reservation (e.g. journal and super-block growth/shrink, LSM tree compaction, and graveyard tombstoning of layer files).

For child stores, extent allocations are not permitted, and extent deallocations are returned to the global allocator pool. This must only be used for child-store transactions that will either not affect net space usage after compaction (e.g. setting attributes on an object) or will reduce space (e.g. unlinking a file or deleting a range of extents).

When this is used appropriately, it is guaranteed that transactions will not fail due to lack of space (barring any bugs resulting in a deficit in the global reservation).

Hold(&'a Reservation)

Use the specified reservation for the transaction’s metadata (placing a hold on it) as well as any allocations or deallocations in the transaction.

Trait Implementations

impl<'a> Clone for ReservationOptions<'a>

fn clone(&self) -> Self

Returns a duplicate of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl<'a> Copy for ReservationOptions<'a>

impl<'a> Default for ReservationOptions<'a>

fn default() -> Self

Returns the “default value” for a type.