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.
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.
RetentionPolicyThe values of
RetentionPolicyused in dCache areREPLICAandCUSTODIAL.REPLICAcorresponds to the lowest quality of the service, usually associated with storing a single copy of each file on the disk.CUSTODIALis 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
DefaultRetentionPolicya retention policy by default. The default value isCUSTODIAL.Edit the file
/etc/dcache/dcache.confto change the default value.Example:
Change the default value to
REPLICA.DefaultRetentionPolicy=REPLICA
AccessLatencyThe two values allowed for
AccessLatencyareNEARLINEandONLINE.NEARLINEmeans 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).ONLINEmeans that data is readily available allowing for faster access.
In case of dCache
ONLINEmeans that there will always be a copy of the file on disk, whileNEARLINEdoes not provide such guarantee. As withRetentionPolicy, once a file is written into a given space reservation, it inherits the reservation’sAccessLatency.If a space reservation request does not specify an access latency we will assign
DefaultAccessLatencyForSpaceReservationan access latency by default. The default value isNEARLINE.Edit the file
/etc/dcache/dcache.confto change the default value.Example:
Change the default value to
ONLINE.DefaultAccessLatencyForSpaceReservation=ONLINE
Important
Please make sure to use capital letters forREPLICA, CUSTODIAL,
ONLINE and NEARLINE
otherwise you will receive an error message.