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.
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
/opt/d-cache/etc/dcache.conf.
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
/opt/d-cache/etc/dcache.conf to change
the default value.
Example:
Change the default value to
REPLICA.
spacemanager.default-retention-policy=REPLICA
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.
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
/opt/d-cache/etc/dcache.conf to change
the default value.
Example:
Change the default value to ONLINE.
spacemanager.default-access-latency=ONLINE
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.
Please make sure to use capital letters for
REPLICA, CUSTODIAL,
ONLINE and NEARLINE
otherwise you will receive an error message.