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Link Groups

The PoolManager supports a type of objects called link groups. These link groups are used by the SRM SpaceManager to make reservations against space. Each link group corresponds to a number of dCache pools in the following way: A link group is a collection of links and each link points to a set of pools. Each link group knows about the size of its available space, which is the sum of all sizes of available space in all the pools included in this link group.

To create a new link group login to the Admin Interface and cd to the PoolManager.

(local) admin > cd PoolManager
(PoolManager) admin > psu create linkGroup <linkgroup>
(PoolManager) admin > psu addto linkGroup <linkgroup> <link>
(PoolManager) admin > save

With save the changes will be saved to the file /var/lib/dcache/config/poolmanager.conf.


You can also edit the file /var/lib/dcache/config/poolmanager.conf to create a new link group. Please make sure that it already exists. Otherwise you will have to create it first via the Admin Interface by

(PoolManager) admin > save

Edit the file /var/lib/dcache/config/poolmanager.conf

psu create linkGroup <linkgroup>
psu addto linkGroup <linkgroup> <link>

After editing this file you will have to restart the domain which contains the PoolManager cell to apply the changes.


Administrators will have to take care, that no pool is present in more than one link group.

Access latency and retention policy.  A space reservation has a retention policy and an access latency, where retention policy describes the quality of the storage service that will be provided for files in the space reservation and access latency describes the availability of the files. See the section called “Properties of Space Reservation” for further details.

A link group has five boolean properties called replicaAllowed, outputAllowed, custodialAllowed, onlineAllowed and nearlineAllowed, which determine the access latencies and retention policies allowed in the link group. The values of these properties (true or false) can be configured via the Admin Interface or directly in the file /var/lib/dcache/config/poolmanager.conf.

For a space reservation to be allowed in a link group, the the retention policy and access latency of the space reservation must be allowed in the link group.

(PoolManager) admin > psu set linkGroup custodialAllowed <linkgroup> <true|false>
(PoolManager) admin > psu set linkGroup outputAllowed <linkgroup> <true|false>
(PoolManager) admin > psu set linkGroup replicaAllowed <linkgroup> <true|false>
(PoolManager) admin > psu set linkGroup onlineAllowed <linkgroup> <true|false>
(PoolManager) admin > psu set linkGroup nearlineAllowed <linkgroup> <true|false>


It is up to the administrator to ensure that the link groups’ properties are specified correctly.

For example dCache will not complain if a link group that does not support a tape backend will be declared as one that supports custodial files.

It is essential that space in a link group is homogeneous with respect to access latencies, retention policies and storage groups accepted. Otherwise space reservations cannot be guaranteed. For instance, if only a subset of pools accessible through a link group support custodial files, there is no guarantee that a custodial space reservation created within the link group will fit on those pools.