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dCache specific concepts

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Activating SRM SpaceManager

In order to enable the SRM SpaceManager you need to add the spacemanager service to your layout file


and add (uncomment) the following definition in the file /opt/d-cache/etc/dcache.conf


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Explicit and Implicit Space Reservations for Data Storage in dCache

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Explicit Space Reservations

Each SRM space reservation is made against the total available disk space of a particular link group. If dCache is configured correctly each byte of disk space, that can be reserved, belongs to one and only one link group. See the section called “SpaceManager configuration for Explicit Space Reservations” for a detailed description.


Make sure that no pool belongs to more than one pool group, no pool group belongs to more than one link and no link belongs to more than one link group.

If a space reservation is specified, the file will be stored in it (assuming the user has permission to do so in the name space).

Files written into a space made within a particular link group will end up on one of the pools belonging to this link group. The difference between the link group’s available space and the sum of all the current space reservation sizes is the available space in the link group.

The total space in dCache that can be reserved is the sum of the available spaces of all link groups.

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Implicit Space Reservations

dCache can perform implicit space reservations for non-SRM transfers, SRM Version 1 transfers and for SRM Version 2.2 data transfers that are not given the space token explicitly. The parameter that enables this behavior is srmImplicitSpaceManagerEnabled, which is described in the section called “SRM configuration for experts”. If no implicit space reservation can be made, the transfer will fail.

In case of SRM version 1.1 data transfers, when the access latency and retention policy cannot be specified, and in case of SRM V2.2 clients, when the access latency and retention policy are not specified, the default values will be used. First SRM will attempt to use the values of AccessLatency and RetentionPolicy tags from the directory to which a file is being written. If the tags are present, then the AccessLatency and RetentionPolicy will be set on basis of the system wide defaults, which are controlled by DefaultRetentionPolicy and DefaultAccessLatencyForSpaceReservation variables in /opt/d-cache/etc/dcache.conf.

You can check if the AccessLatency and RetentionPolicy tags are present by using the following commands:

[root] # /opt/d-cache/bin/ lstag /path/to/directory
Total: numberOfTags

If the output contains the lines AccessLatency and RetentionPolicy then the tags are already present and you can get the actual values of these tags by executing the following commands, which are shown together with example outputs:


[root] # /opt/d-cache/bin/ readtag /data/experiment-a AccessLatency
[root] # /opt/d-cache/bin/ readtag /data/experiment-a RetentionPolicy

The valid AccessLatency values are ONLINE and NEARLINE, valid RetentionPolicy values are REPLICA and CUSTODIAL.

To create/change the values of the tags, please execute :

[root] # /opt/d-cache/bin/ writetag /path/to/directory AccessLatency "<New AccessLatency>"
[root] # /opt/d-cache/bin/ writetag /path/to/directory RetentionPolicy "<New RetentionPolicy>"


Some clients also have default values, which are used when not explicitly specified by the user. I this case server side defaults will have no effect.


If the implicit space reservation is not enabled the pools in the link groups will be excluded from consideration and only the remaining pools will be considered for storing the incoming data, and classical pool selection mechanism will be used.