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Utilization of Space Reservations for Data Storage

SRM version 2.2 introduced a concept of space reservation. Space reservation guarantees that the requested amount of storage space of a specified type is made available by the storage system for a specified amount of time.

The dCache administrator can make space reservations for VOs (see the section called “SpaceManager configuration for Explicit Space Reservations”. Each space reservation has an associated ID (or space token). VOs then can copy directly into space tokens assigned to them by the dcache administrator.

When a file is about to be transferred to a storage system, the space available in the space reservation is checked if it can accomodate the entire file. If yes, this chunk of space is marked as allocated, so that it can not be taken by another, concurrently transferred file. If the file is transferred successfully the allocated space becomes used space within the space reservation, else the allocated space is released back to the space reservation.

SRM space reservation can be assigned a non-unique description which can be used to query the system for space reservations with a given description.

dCache only manages write space, i.e. space on disk can be reserved only for write operations. Once files are migrated to tape, and if no copy is required on disk, space used by these files is returned back into space reservation. When files are read back from tape and cached on disk, they are not counted as part of any space.

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Properties of Space Reservation

The administrator can specify a RetentionPolicy and an AccessLatency for the space reservation.

RetentionPolicy describes the quality of the storage service that will be provided for the data (files) stored in this space reservation and AccessLatency describes the availability of this data. The specification requires that if a space reservation is given, then the specified RetentionPolicy or AccessLatency must match those of the space reservation.

The default values for the RetentionPolicy and AccessLatency can be changed in the file /etc/dcache/dcache.conf.


The values of RetentionPolicy used in dCache are REPLICA and CUSTODIAL.

  • REPLICA corresponds to the lowest quality of the service, usually associated with storing a single copy of each file on the disk.

  • CUSTODIAL is the highest quality service, usually interpreted as storage of the data on tape.

Once a file is written into a given space reservation, it inherits the reservation’s RetentionPolicy.

If the space reservation request does not specify a retention policy we will assign DefaultRetentionPolicy a retention policy by default. The default value is CUSTODIAL.

Edit the file /etc/dcache/dcache.conf to change the default value.


Change the default value to REPLICA.


The two values allowed for AccessLatency are NEARLINE and ONLINE.

  • NEARLINE means that data stored in this reservation is allowed to migrate to permanent media. Retrieving these data may result in delays associated with preparatory steps that the storage system has to perform to make these data available for the user I/O (e.g., staging data from tape to a disk cache).

  • ONLINE means that data is readily available allowing for faster access.

In case of dCache ONLINE means that there will always be a copy of the file on disk, while NEARLINE does not provide such guarantee. As with RetentionPolicy, once a file is written into a given space reservation, it inherits the reservation’s AccessLatency.

If a space reservation request does not specify an access latency we will assign DefaultAccessLatencyForSpaceReservation an access latency by default. The default value is NEARLINE.

Edit the file /etc/dcache/dcache.conf to change the default value.


Change the default value to ONLINE.



Please make sure to use capital letters for REPLICA, CUSTODIAL, ONLINE and NEARLINE otherwise you will receive an error message.