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 a detailed description.

If a space reservation is specified during upload, the file will be stored in it.

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 free space and the sum of all its space reservation’s unused space is the available space of the link group. The available space of a link group is the space that can be allocated for new space reservations.

The total space in dCache that can be reserved is the sum of the available spaces of all link groups. Note however that a space reservation can never span more than a single link group.

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 srm.enable.space-reservation.implicit, which is described in the section called “SRM configuration for experts”. If no implicit space reservation can be made, the transfer will fail.

Implicit space reservation means that the srm will create a space reservation for a single upload while negotiating the transfer parameters with the client. The space reservation will be created in a link group for which the user is authorized to create space reservations, which has enough available space, and which is able to hold the type of file being uploaded. The space reservation will be short lived. Once it expires, it will be released and the file it held will live on outside any space reservation, but still within the link group to which it was uploaded. Implicit space reservations are thus a technical means to upload files to link groups without using explicit space reservations.

The reason dCache cannot just allow the file to be uploaded to the link group without any space reservation at all is, that we have to guarantee, that space already allocated for other reservations isn’t used by the file being uploaded. The best way to guarantee that there is enough space for the file is to make a space reservation to which to upload it.

In case of SRM version 1.1 data transfers, where 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, default values will be used. First SRM will attempt to use the values of access latency and retention policy tags from the directory to which a file is being written. If the tags are not present, then the access latency and retention policy will be set on basis of pnfsmanager defaults controlled by pnfsmanager.default-retention-policy and pnfsmanager.default-access-latency variables in /etc/dcache/dcache.conf.

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

[root] # /usr/bin/chimera lstag /path/to/directory
Total: numberOfTags
tag1
tag2
..
AccessLatency
RetentionPolicy

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] # /usr/bin/chimera readtag /data/experiment-a AccessLatency
ONLINE
[root] # /usr/bin/chimera readtag /data/experiment-a RetentionPolicy
CUSTODIAL

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] # /usr/bin/chimera writetag /path/to/directory AccessLatency "<New AccessLatency>"
[root] # /usr/bin/chimera writetag /path/to/directory RetentionPolicy "<New RetentionPolicy>"