THE PINMANAGER SERVICE
The purpose of the pinmanager service in dCache is ensuring the presence of a file replica on disk.
It can be used by explicitly (un)pinning files via the admin interface or the bulk service, but is also used by the resilience component to ensure having a certain number of replicas available. PinManager is also used for keeping a replica online after fetching (staging) it from a connected tertiary storage system (sometimes called HSM - hierarchical storage manager), if staging is allowed.
(Un-)Pinning Concept
A pin, also called sticky-ness, is a concept describing a file replica on a pool that cannot be deleted for a certain duration. The pin effectively suppresses automatic garbage collection for the lifetime of the pin.
Pins may have a finite or infinite lifetime. Pins also have an owner, which may be a dCache service (such as resilience) or client through a protocol such as srm. Only the owner is allowed to remove unexpired pins. Several pins (for different users) can exist for the same pnfsid, and a file is considered pinned as long as at least one unexpired pin exists.
The Pin Life Cycle
When a pin is created, it will initially appear in state PINNING, then transition to state PINNED once the attempt is successful.
When a pin either has a finite lifetime that has expired or is directly requested to be removed, it is put into state READY_TO_UNPIN. An ‘unpinning’ background task runs regularly (default every minute), which selects a certain number of pins (default 200) in state READY_TO_UNPIN and attempts to remove them, during which the pins are in state UNPINNING.
On success, the pin is deleted from the pool in question as well as the database, of failure the pin is put into state FAILED_TO_UNPIN. Another background process regularly (default every 2h) resets all pins in state FAILED_TO_UNPIN back to state READY_TO_UNPIN in order to make them eligible to be attempted again.
Configuring
The PinManager service can be run in a shared domain. It may also be deployed in high availability (HA) mode (coordinated via ZooKeeper) by having several PinManager cells in a dCache instance, which then need to share the same database and configuration.
pinmanager.db.host=pinman-db-hostname
pinmanager.db.name=dcache
pinmanager.db.password=
pinmanager.db.user=dcache
Pins are managed in this central database as well as on the pools containing the replicas.
Pin expiration and pin unpinning are background tasks which are executed regularly. The property pinmanager.expiration-period controls how often to execute these tasks. The default value is 60 seconds.
The number of pins that should at most be attempted to be removed per unpinning task run can be configured with the property pinmanager.max-unpins-per-run and default to 200. A value of -1 indicates that there is no limit on the number of pins the PinManager will attempt to unpin per run, which might lead to large CPU and memory loads if there are many pending unpin operations.
Another background task takes care of resetting pins that previously failed to be removed. It can be configured via pinmanager.reset-failed-unpins-period and defaults to 2h.