SLD Staging system on the SLACVX cluster
Staging is a mechanism by which tape data is first copied to disk
and then read from disk by the user. The advantage of this over
directly reading the tape is that
- The tape drive is only tied up during the relatively fast
tape-to-disk copy, not for the entire duration of the users
- If the data is to be read many times it only needs to be
copied to disk the first time around.
The SLD staging system was designed as a general purpose staging
system for VMS. Although in principle the system can be used to stage
any type of tape data, currently the only supported way to use the
system is for staging input JAZZDATA files via IDA.
The staging system maintains a large pool of disk space where
the staged data is stored. Data is kept on disk as long as space
if available, but when space is needed for new files, the least
recently used files are deleted from disk automatically.
How to use the staging system
To read files with the staging system from IDA all you have to do is
use the IDA
OPENTAPE command. Two extra qualifiers control staging:
- Tells IDA you want to read the file using the STAGING system
rather than directly reading the tape.
- Tells IDA you want to wait for the data to be put on disk. If
you do not specify WAIT when running IDA interactively and the data is not
already on disk, the the job to copy the data to disk is submitted, but the
OPENTAPE command is then terminated with an error. If you specify WAIT then
the OPENTAPE command will wait until the data has been copied to disk before
When running in batch WAIT is the default.
Data is actually copied to disk by a batch job submitted under the userid
SLDSTAGE. You will be able to see such jobs from
time-to-time in the batch system, they can be idenified by the fact that they
are always named STAGE_XXXXXX, where XXXXX is the tape name (e.g.QW1234).
If for any reason that input staging job fails, you might need to look at the
log files for the staging job. This can be found as
Other Useful Commands
The STAGE/QUERY command can be used to check the status of
staged files. You can also use WWW to query the
status of the staging system.
The STAGE/CLEAR command can be used to clear files out of
the staging area.
Read the source code (in STAGE_$SRC).
For more information you could try reading the