Database Reference
In-Depth Information
to be in Tape1.
*
A file residing on an experiment-managed disk is said to be
in Disk1. Tape0 means that the file does not have a copy stored on a reliable
storage system. Disk0 means that the disk where the copy of the file resides
is managed by the system: If such a copy is not being used, the system can
delete it.
The Storage Classes Working Group decided that only certain combinations
(or storage classes) are needed for the time being, corresponding to specific
choices for the Retention Policy and the Access Latency as defined by SRM
v2.2:
=
Custodial-Near-line
Tape1Disk0 class
Custodial-Online
=
Tape1Disk1 class
Replica-Online
=
Tape0Disk1 class
Tape0Disk0 is not implemented. It is pure scratch space that could be
emulated using one of the available classes and removing the data explicitly
once done. However, it could be handy for LHC VOs to have such a type of
space actually implemented eventually.
In the
custodial-near-line
storage class data is stored on some reliable
secondary storage system (such as a robotic tape or DVD library). Access
to data may imply significant latency. In WLCG this means that a copy
of the file is on tape (Tape1). When a user accesses a file, the file is re-
called in a cache that is managed by the system (Disk0). The file can be
pinned for the time the application needs the file. However, the treatment of
a pinned file on a system-managed disk is implementation dependent, some
implementations choosing to honor pins until they expire or are released,
and others removing unused online copies of files to make space for new
requests.
In the
custodial-online
storage class, data is always available on disk. A
copy of the data resides permanently on tape, DVD, or on a high-quality
RAID system as well. The space owner (the virtual organization) manages
the space available on disk. If no space is available in the disk area for a new
file, the file creation operation fails. This storage class guarantees that a file
is never removed by the system.
The
replica-online
storage class is implemented through the use of disk-
based solutions not necessarily of high quality. The data resides on disk space
managed by the virtual organization.
Through the storage system interface, it is possible to schedule storage
class transitions for a list of files. Only the following transitions are allowed
in WLCG:
*
The term Tape1 was coined to mean that a single copy of the file is residing on tape. Accordingly,
it is possible to have Tape2 (two copies), Tape3 (three copies), and so forth, and likewise for Disk1.
However, it was decided to avoid the ability to specify multiple copies at the present time.
