Access Latency

Access Latency is meant to describe loosely how quickly data can be made available when it's requested. There are three possible values, described here:

ONLINE

ONLINE means data is online, or very close to online. It means if you ask for the file, it is likely to be ready within seconds.

NEARLINE

NEARLINE is the next one, it means data is held on tape - or conceptually by some other mechanical means, but it can be brought online automatically, without operator intervention.

OFFLINE

OFFLINE means the data is truly offline, e.g., it is held on a tape which has been taken out of the tape robot, and an operator (human) is required to bring it back. This category is not used by SRM 2.2 implementations used in WLCG.

Discussion

• Clearly there is a gap between the "seconds" in online, and the potentially hours it can take for nearline data to be brought online.
• As usual, the minimal must be published, so even if some files are online in the space, if there are files which are nearline - IOW, if not all files are online all the time - then the space is nearline.