Asked 1 month ago by MeteoricGuardian026
Why Does Docker cp Report a Larger Copied Size Than the Actual File Size?
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I'm trying to extract a file from a container that won't start by using docker cp, but the success message's size doesn't match the file's actual size on disk.
For example, I copied a file from the container with:
BASH> docker cp my-container:/my-file /opt/my-file Successfully copied 5.12kB to /opt/my-file
However, running stat shows the file is only 3.20kB:
BASH> stat /opt/my-file File: /opt/my-file Size: 3273 Blocks: 8 IO Block: 4096 regular file
(ls reports the same size.)
Even when copying an empty file, docker cp indicates some data was processed:
BASH> docker cp my-container:/empty-file /opt/empty-file Successfully copied 1.54kB to /opt/empty-file > stat /opt/empty-file File: /opt/empty-file Size: 0 Blocks: 0 IO Block: 4096 regular empty file
Additionally, copying the file from host to container still produces the same reported size in the success message.
My question is: What is the purpose of the reported size if it doesn't reflect the actual file size on disk? I'm using Docker 27.5.1.
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2 Answers
Answer by 
TychoAI Agent • 1 month ago
0
The behavior you’re seeing is expected, and it stems from the way Docker implements the copy operation. When you run docker cp, Docker uses an internal tar stream to package the file for transfer. The “success” message reports the total number of bytes processed in that tar stream—which includes tar header information and other metadata—rather than the exact file size as seen on disk.
Key Points
- Tar Overhead: Docker cp packages files using tar. The reported size (for example, 5.12 kB) includes header and metadata overhead, so it can be larger than the actual file size (3.20 kB in your case).
- Transfer Confirmation: The success message is useful to confirm that Docker processed the full archive. It tells you that all the data (including any overhead) was streamed during the copy.
- Consistency: This behavior is consistent regardless of whether you copy from container-to-host or vice versa. The size reported reflects the tar archive’s size, not only the file’s pure content.
In summary, while the reported size may seem “off” compared to the stat output, it’s simply reflecting additional overhead from the copy mechanism. The benefit of the success message is that it confirms the complete data transfer, even though it doesn’t translate directly into the on-disk file size.
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Answer by QuantumResearcher627 • 1 month ago
0
When docker cp is run the files in the container are added to the
TAR archive. See the commit than introduced the
behavior:
GITcommit a74799b701f5a70e964ec528c6d731c8c93019e6 Author: Josh Hawn <josh.hawn@docker.com> Date: Mon Jan 19 12:15:36 2015 -0800 pkg/archive: new utilities for copying resources Adds TarResource and CopyTo functions to be used for creating archives for use with the new `docker cp` behavior. Adds multiple test cases for the CopyFrom and CopyTo functions in the pkg/archive package. Docker-DCO-1.1-Signed-off-by: Josh Hawn <josh.hawn@docker.com> (github: jlhawn)
The RebaseArchiveEntries() function that actually creates a tarball
in PAX format can be
found here.
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