Prestage

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Introduction

In the process of uploading files to tape, they are copied to a tape-backed dCache disk area. From there, they migrate automatically to tape and after that, the least-used will be deleted from the dCache when disk space is needed.

Files existing only on tape will be copied to the dCache disk if a user attempts to access the file through its /pnfs filespec. It takes up to one minute or more to mount a tape and find a random file. Backlogs at times of high demand on tape drives can cause hours of wait time.

Prestaging is the process of making sure all the files in a dataset have been copied off tape and written back to disk in dCache so they are ready to be used in a grid job.

When to prestage

First, when in doubt, prestage. It is harmless, except for the delay, and you can be confident that tape response will not be a problem.

Next, you can check if the files are on disk. You can run the SAM utility script samOnDisk:

setup dhtools
samOnDisk DATASET

on a dataset name. This script selects some files at random and check if they are on disk. After a few minutes it should become clear what fraction are on disk. If it is nearly 100%, you don't need to prestage.

You do not need to prestage a dataset if it is less than a few hundred files. in this case the system should respond in time so that your grid job will succeed. Note that the prestage should also be quick, so still a good idea to run.

Prestage less than 100K files

As a practical matter, it is better to handle larger dataset by splitting them up first. Smaller datasets, less than 100K files, can be prestaged in one command. You can see how many files in your dataset with:

samweb count-files "dh.dataset=DATASET"

where DATASET is a mu2e SAM dataset name. Prestage with

setup dhtools
samweb prestage-dataset --parallel=5 --defname=DATASET

The prestage will create a SAM project on the SAM station and create consumers to start requesting files from the project. The project on the SAM station has a knowledge of all the files it will need, so it does two things:

• gives out the files it thinks are more likely to be on disk first
• look ahead in the file list and start prestaging upcoming files in a logical and efficient manner

The first point can be seen as the prestaging proceeding quickly as long it keeps finding files on disk, then slowing down when it starts requesting files off tape.

Prestage speed

Overall the prestage speed is about 100K files per day if things are going well. If the project has to get files off tape, and enstore is very busy, it may slow down by factors of 2 or 3. There may be periods of several minutes when the file count does not progress. You can monitor the progress of the SAM station by the SAM station links on the ComputingOps page.