Glossary
The following terms are used in dCache.
The dcache.conf File
This is the primary configuration file of a dCache. It is located at /etc/dcache/dcache.conf.
The dcache.conf file is initially empty. If one of the default configuration values needs to be changed, copy the default setting of this value from one of the properties files in /usr/share/dcache/defaults to this file and update the value.
The layout File
The layout file is located in the directory /etc/dcache/layouts. It contains lists of the domains and the services that are to be run within these domains.
The properties files are located in the directory /usr/share/dcache/defaults. They contain the default settings of the dCache.
Chimera
The Chimera namespace is a core component of dCache. It maps each stored file to a unique identification number and allows storing of metadata against either files or directories.
Chimera includes some features like levels, directory tags and many of the dot commands.
Chimera ID
A Chimera ID is a 36 hexadecimal digit that uniquely defines a file or directory.
Domain
A domain is a collection of one or more cells that provide a set of related services within a dCache instance. Each domain requires its own Java Virtual Machine. A typical domain might provide external connectivity (i.e., a door) or manage the pools hosted on a machine.
Each domain has at least one cell, called the System cell and many tunnel cells for communicating with other Domains. To provide a useful service, a domain will contain other cells that provide specific behaviour.
Cell
A cell is a collection of Java threads that provide a discrete and simple service within dCache. Each cell is hosted within a domain.
Cells have an address derived from concatenating their name, the @ symbol and their containing domain name.
Well Known Cell
A well-known cell is a cell that registers itself centrally. Within the admin interface, a well-known cell may be referred to by just its cell name.
Door
Door is the generic name for special cells that provide the first point of access for end clients to communicate with a dCache instance. There are different door implementations (e.g., GSIdCap door and GridFTP door), allowing a dCache instance to support multiple communication protocols.
A door will (typically) bind to a well-known port number depending on the protocol the door supports. This allows for only a single door instance per machine for each protocol.
A door will typically identify which pool will satisfy the end user’s operation and redirect the client to the corresponding pool. In some cases this is not possible; for example, some protocols (such as GridFTP version 1) do not allow servers to redirect end-clients, in other cases pool servers may be behind a firewall, so preventing direct access. When direct end-client access is not possible, the door may act as a data proxy, streaming data to the client.
By default, each door is hosted in a dedicated domain. This allows easy control of whether a protocol is supported from a particular machine.
Java Virtual Machine (JVM)
Java programs are typically compiled into a binary form called Java byte-code. Byte-code is comparable to the format that computers understand native; however, no mainstream processor understands Java byte-code. Instead compiled Java programs typically require a translation layer for them to run. This translation layer is called a Java Virtual Machine (JVM). It is a standardised execution environment that Java programs may run within. A JVM is typically represented as a process within the host computer.
tertiary storage system
A mass storage system which stores data and is connected to the dCache system. Each dCache pool will write files to it as soon as they have been completely written to the pool (if the pool is not configured as a LFS). The tertiary storage system is not part of dCache. However, it is possible to connect any mass storage system as tertiary storage system to dCache via a simple interface.
tape backend
A tertiary storage system which stores data on magnetic tapes.
Hierarchical Storage Manager (HSM)
HSM Type
The type of HSM which is connected to dCache as a tertiary storage system. The choice of the HSM type influences the communication between dCache and the HSM. Currently there are osm and enstore. osm is used for most HSMs (TSM, HPSS, …).
HSM Instance
Large File Store (LFS)
A Large File Store is the name for a dCache instance that is acting as a filesystem independent of, or in cooperation with, an HSM system. When dCache is acting as an LFS, files may be stored and later read without involving any HSM system.
Whether a dCache instance provides an LFS depends on whether there are pools configured to do so. The LFS option, specified for each pool within the layout file, describes how that pool should behave. This option can take three possible values:
-
none or hsm - the pool does not contribute to any LFS capacity. All newly written files are regarded precious and sent to the HSM backend.
-
precious - Newly created files are regarded as precious but are not scheduled for the HSM store procedure. Consequently, these file will only disappear from the pool when deleted in the Chimera namespace.
to store
Copying a file from a dCache pool to the tertiary storage system.
to restore
Copying a file from the tertiary storage system to one of the dCache pools.
to stage
See to restore.
transfer
Any kind of transfer performed by a dCache pool. There are store, restore, pool to pool (client and server), read, and write transfers. The latter two are client transfers. See Also mover.
mover
The process/thread within a pool which performs a transfer. Each pool has a limited number of movers that may be active at any time; if this limit is reached then further requests for data are queued.
In many protocols, end clients connect to a mover to transfer file contents. To support this, movers must speak the protocol the end client is using. See Also transfer.
Location Manager
The location manager is a cell that instructs newly started domains to which domains they should connect. This allows domains to form arbitrary network topologies; although, by default, a dCache instance will form a star topology with the dCacheDomain domain at the centre.
Pinboard
The pinboard is a collection of messages describing events within dCache and is similar to a log file. Each cell will (typically) have its own pinboard.
Breakeven Parameter
The breakeven parameter has to be a positive number smaller than 1.0. It specifies the impact of the age of the least recently used file on space cost. It the LRU file is one week old, the space cost will be equal to (1 + breakeven). Note that this will not be true, if the breakeven parameter has been set to a value greater or equal to 1.
least recently used (LRU) file
The file that has not be requested for the longest.
file level
In Chimera, each file can have up to eight independent contents; these file-contents, called levels, may be accessed independently. dCache will store some file metadata in levels 1 and 2, but dCache will not store any file data in Chimera.
directory tag
Chimera includes the concept of tags. A tag is a keyword-value pair associated with a directory. Subdirectories inherit tags from their parent directory. New values may be assigned, but tags cannot be removed. The dot command .(tag)(
More details on directory tags are given in the section called “Directory Tags”.
dot command
To configure and access some of the special features of the Chimera namespace, special files may be read, written to or created. These files all start with a dot (or period) and have one or more parameters after. Each parameter is contained within a set of parentheses; for example, the file .(tag)(
Care must be taken when accessing a dot command from a shell. Shells will often expand parentheses so the filename must be protected against this; for example, by quoting the filename or by escaping the parentheses.
Wormhole
A wormhole is a feature of the Chimera namespace. A wormhole is a file that is accessible in all directories; however, the file is not returned when scanning a directory (e.g., using the ls command).
Pool to Pool Transfer
A pool-to-pool transfer is one where a file is transferred from one dCache pool to another. This is typically done to satisfy a read request, either as a load-balancing technique or because the file is not available on pools that the end-user has access to.
Storage Class
The storage class is a string of the form
<StoreName>:<StorageGroup>@<type-of-storage-system>
containing exactly one @-symbol.
<StoreName>:<StorageGroup> is a string describing the storage class in a syntax which depends on the storage system.
<type-of-storage-system> denotes the type of storage system in use.
In general use <type-of-storage-system>=osm.
A storage class is used by a tertiary storage system to decide where to store the file (i.e. on which set of tapes). dCache can use the storage class for a similar purpose, namely to decide on which pools the file can be stored.
Replica
It is possible that dCache will choose to make a file accessible from more than one pool using a pool-to-pool copy. If this happens, then each copy of the file is a replica.
A file is independent of which pool is storing the data whereas a replica is uniquely specified by the pnfs ID and the pool name it is stored on.
Precious Replica
A precious replica is a replica that should be stored on tape.
Cached Replica
A cached replica is a replica that should not be stored on tape.
Resilience
Resilience has to do with file preservation and availabilitykeeps. The Resilience service or manager tracks the number of replicas of files within defined subsets of pools (marked as resilient) and makes sure this number is always as specified by the storage class of the file. This is especially useful to ensure the existence or readability of the file, even if the hardware is not reliable, if a high volume of access to such files is foreseen, or if the file should be permanent but not reside on tertiary storage. The activation and configuration of resilience is described in Chapter 6, The Resilience Service.
Storage Resource Manager (SRM)
An SRM provides a standardised webservice interface for managing a storage resource (e.g. a dCache instance). It is possible to reserve space, initiate file storage or retrieve, and replicate files to another SRM. The actual transfer of data is not done via the SRM itself but via any protocol supported by both parties of the transfer. Authentication and authorisation is done with the grid security infrastructure. dCache comes with an implementation of an SRM which can turn any dCache instance into a grid storage element.
Billing/Accounting
Accounting information is either stored in a text file or in a PostgreSQL database by the billing cell usually started in the domain httpdDomain. This is described in Chapter 15, The billing Service.
Pool Manager
The pool manager is the cell running in the domain dCacheDomain. It is a central component of a dCache instance and decides which pool is used for an incoming request.
Cost Module
The cost module is a Java class responsible for combining the different types of cost associated with a particular operation; for example, if a file is to be stored, the cost module will combine the storage costs and CPU costs for each candidate target pool. The pool manager will choose the candidate pool with the least combined cost.
Pool Selection Unit
The pool selection unit is a Java class responsible for determining the set of candidate pools for a specific transaction. A detailed account of its configuration and behaviour is given in the section called “The Pool Selection Mechanism”.
Pin Manager
The pin manager is a cell by default running in the utility domain. It is a central service that can “pin” files to a pool for a certain time. It is used by the SRM to satisfy prestage requests.
Space Manager
The (SRM) Space Manager is a cell by default running in the srm domain. It is a central service that records reserved space on pools. A space reservation may be either for a specific duration or never expires. The Space Manager is used by the SRM to satisfy space reservation requests.
Pool
A pool is a cell responsible for storing retrieved files and for providings access to that data. Data access is supported via movers. A machine may have multiple pools, perhaps due to that machine’s storage being split over multiple partitions.
A pool must have a unique name and all pool cells on a particular machine are hosted in a domain that derives its name from the host machine’s name.
The list of directories that are to store pool data are found in the definitions of the pools in the layout files, which are located on the pool nodes.
sweeper
A sweeper is an activity located on a pool. It is responsible for deleting files on the pool that have been marked for removal. Files can be marked for removal because their corresponding namespace entry has been deleted or because the local file is a cached replica and more disk space is needed.
HSM sweeper
The HSM sweeper, if enabled, is a component that is responsible for removing files from the HSM when the corresponding namespace entry has been removed.
cost
The pool manager determines the pool used for storing a file by calculating a cost value for each available pool. The pool with the lowest cost is used. The costs are calculated by the cost module as described in the section called “Classic Partitions”. The total cost is a linear combination of the performance cost and the space cost. I.e.,
cost = ccf * performance_cost + scf * space_cost
where ccf and scf are configurable with the command set pool decision.
performance cost
See also the section called “The Performance Cost”.
space cost
See also the section called “The Space Cost”.