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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.

Users can create space reservations using an appropriate SRM client, although it is more common for the dCache administrator to make space reservations for VOs (see the section called “SpaceManager configuration”. 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 as free space.

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

A space reservation has a retention policy and an access latency.

Retention policy describes the quality of the storage service that will be provided for the data (files) stored in the space reservation and access latency describes the availability of this data. The SRM specification requires that if a space reservation is given on upload, then the specified retention policy and access latency must match those of the space reservation.

The default values for the retention policy and access latency can be changed in the file /etc/dcache/dcache.conf.

Retention policy

The values of retention policy supported by 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 retention policy.

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

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

Example:

Change the default value to REPLICA.

spacemanager.default-retention-policy=REPLICA

Note

spacemanager.default-retention-policy merely specifies to value to use while allocating space reservations when no value was given by the client or dCache admin. It is not to be confused with pnfsmanager.default-retention-policy which specifies the default retention policy of files uploaded outside of any space reservation.

Access latency

The two values allowed for access latency 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 retention policy, once a file is written into a given space reservation, it inherits the reservation’s access latency.

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

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

Example:

Change the default value to ONLINE.

spacemanager.default-access-latency=ONLINE

Note

spacemanager.default-access-latency merely specifies to value to use while allocating space reservations when no value was given by the client or dCache admin. It is not to be confused with pnfsmanager.default-access-latency which specifies the default retention policy of files uploaded outside of any space reservation.

Important

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