Why copying hurts
Last updated on 2026-10-27 | Edit this page
Estimated time: 10 minutes
Overview
Questions
- Why does a directory of small files copy slowly?
- Which directory is for the heavy copy, and which one exists only inside a job?
Objectives
- Separate the work of the data servers from the work of the metadata server.
- Put code in home, the heavy copy on scratch, long-lived group data on project, and node-disk files only inside a job.
A million small files
A parallel file system splits the work. Data servers move bytes. A metadata server records each create, open, and close. Copying a million small files asks that server for a million operations, and the data servers wait. Pack the files first.
Sam’s case is the same shape at a smaller count. Thousands of output files, copied one by one, stall on metadata. The next episode packs 20,000 of them and times the difference.
Where data lives
Keep code and notes in home. Do the heavy copy on scratch. Keep the group’s long-lived data on the project file system. Use the node disk only inside a job. It is the fastest, and it disappears when the job ends.
The paths differ by machine.
- Data servers move bytes. The metadata server records create, open, and close.
- A million small files stall the copy. Pack them first.
- Home is for code, scratch is for the heavy copy, project is for long-lived group data.
- The node disk is only for the inside of a job.