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
/opt/d-cache/etc/dcache.conf.
RetentionPolicy
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
pnfsmanager.default-retention-policy a retention
policy by default. The default value is
CUSTODIAL.
Edit the file
/opt/d-cache/etc/dcache.conf to change
the default value.
Example:
Change the default value to
REPLICA.
pnfsmanager.default-retention-policy=REPLICA
AccessLatency
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
spacemanager.default-access-latency an
access latency by default. The default value is
NEARLINE.
Edit the file
/opt/d-cache/etc/dcache.conf to change
the default value.
Example:
Change the default value to ONLINE.
spacemanager.default-access-latency=ONLINE
REPLICA, CUSTODIAL,
ONLINE and NEARLINE
otherwise you will receive an error message.