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
/etc/dcache/dcache.conf.
- Retention policy
The values of retention policy supported by dCache are
REPLICAandCUSTODIAL.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 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 isCUSTODIAL.Edit the file
/etc/dcache/dcache.confto change the default value.Example:
Change the default value to
REPLICA.spacemanager.default-retention-policy=REPLICA
Note
spacemanager.default-retention-policymerely 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 withpnfsmanager.default-retention-policywhich specifies the default retention policy of files uploaded outside of any space reservation.- Access latency
The two values allowed for access latency are
NEARLINEandONLINE.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 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 isNEARLINE.Edit the file
/etc/dcache/dcache.confto change the default value.Example:
Change the default value to
ONLINE.spacemanager.default-access-latency=ONLINE
Note
spacemanager.default-access-latencymerely 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 withpnfsmanager.default-access-latencywhich 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.