ReferenceLogging

Logging

Where s3mirror's output goes for each combination of flags, the format of the console and the log file, and what each line means.

s3mirror logs to two places: the console, always, and a file, when you pass --log-file. The console level depends on the flags; the file always gets everything.

Console level

Flags	Console shows	Line format
none	Progress and the summary (info and above)	The message alone
`--debug`	Everything (debug and above)	`[LEVEL] message`
`--quiet`	Errors only	The message alone
`--quiet --debug`	Errors only	`[LEVEL] message`
`--log-file`	Errors only	The message alone
`--log-file --debug`	Everything	`[LEVEL] message`
`--log-file --quiet`	Errors only	The message alone

--quiet wins over --debug for the level, but --debug still adds the level prefix. When standard output is a terminal, --debug also colours the level names.

Every log line goes to standard output, errors included. Three things go to standard error instead, because they happen before logging starts: config file errors (Error: Config file not found: ... and the like), usage errors from bad flags, and a Python traceback if the log file can’t be opened.

The log file

With --log-file, every line at debug level and above is appended to the file, with a timestamp and the level padded to eight characters. Each run starts with a session header:

2026-11-05 08:10:55 [INFO    ]
2026-11-05 08:10:55 [INFO    ] ######################################################################
2026-11-05 08:10:55 [INFO    ] # NEW SESSION: 2026-11-05 08:10:55
2026-11-05 08:10:55 [INFO    ] ######################################################################
2026-11-05 08:10:55 [INFO    ] ======================================================================
2026-11-05 08:10:55 [INFO    ] S3 BUCKET MIRROR
2026-11-05 08:10:55 [INFO    ] Session started: 2026-11-05 08:10:55
2026-11-05 08:10:55 [INFO    ] ======================================================================
2026-11-05 08:10:55 [DEBUG   ] ======================================================================
2026-11-05 08:10:55 [DEBUG   ] INITIALIZATION
2026-11-05 08:10:55 [DEBUG   ] ======================================================================
2026-11-05 08:10:55 [DEBUG   ] Source endpoint: https://s3.source.example.com

The file is never truncated or rotated by s3mirror. It’s opened when the run starts and closed when it ends, so any log rotation tool can rotate it between runs.

A bucket at debug level

This is one bucket from a log file, from the session above: one object changed size, two are new, and one exists only at the destination.

2026-11-05 08:10:55 [INFO    ]   Source: 4 objects
2026-11-05 08:10:55 [INFO    ]   Destination: 3 objects
2026-11-05 08:10:55 [DEBUG   ]   Calculating differences...
2026-11-05 08:10:55 [DEBUG   ]     Changed: config/settings.json (src:2.3KB != dst:2.1KB)
2026-11-05 08:10:55 [DEBUG   ]     New file: exports/2026-11-02.csv (49.8KB)
2026-11-05 08:10:55 [DEBUG   ]     New file: exports/2026-11-03.csv (48.7KB)
2026-11-05 08:10:55 [DEBUG   ]   Files to delete: 1
2026-11-05 08:10:55 [DEBUG   ]     - exports/old-report.csv
2026-11-05 08:10:55 [INFO    ]   Actions: 3 to copy (100.8KB), 1 to delete
2026-11-05 08:10:55 [DEBUG   ]   Starting parallel copy (20 workers)...
2026-11-05 08:10:55 [DEBUG   ]     ✓ config/settings.json [2.3KB]
2026-11-05 08:10:55 [DEBUG   ]     ✓ exports/2026-11-02.csv [49.8KB]
2026-11-05 08:10:55 [DEBUG   ]     ✓ exports/2026-11-03.csv [48.7KB]
2026-11-05 08:10:55 [INFO    ]   ✓ Copied 3 objects in 0.0s (45.0MB/s)
2026-11-05 08:10:55 [DEBUG   ]   Starting parallel delete (20 workers)...
2026-11-05 08:10:55 [DEBUG   ]     ✓ Deleted: exports/old-report.csv
2026-11-05 08:10:55 [INFO    ]   ✓ Deleted 1 objects in 0.0s
2026-11-05 08:10:55 [INFO    ]   ✓ Bucket completed in 0.0s
2026-11-05 08:10:55 [DEBUG   ]   Bucket stats: copied=3, deleted=1, errors=0

Files to delete lists at most five keys, followed by ... and N more; the ✓ Deleted: lines name every key. In Bucket stats, copied is for this bucket, but deleted and errors are totals for the run so far.

What each level holds

Info is the run’s progress: the banner, the two connection checks, the excluded buckets, then for each bucket its [n/N]: name header, a Created bucket line if it was created, the object counts on each side, the Actions: line, the copy and delete totals with their timings, or Already synchronized (no changes needed), and the bucket’s time. Then the summary.

Warning marks a risk or something that didn’t happen: ⚠ TLS certificate verification is off for the source (or destination), No buckets found to sync, Skipping bucket due to HeadBucket failure, due to creation failure or due to listing failure, ⚠ N objects failed to copy or failed to delete, ⚠ Errors encountered: N in the summary, Stopping: dropping the queued work ..., Operation cancelled by user (Ctrl+C) and Operation stopped by SIGTERM.

Error is a failed call: the connection check (✗ Source connection FAILED with the error code, message and HTTP status), Failed to list source buckets, Failed to check bucket on destination, Failed to create bucket, Failed to list objects in <bucket> on the source or on the destination, Failed to copy <key>, Failed to delete <key>, and FATAL ERROR with a traceback for anything unexpected.

Debug adds the detail: the settings in use, including each endpoint’s region, with access key IDs shortened to their first and last four characters (or *** if shorter than 20), a sample of the source’s bucket names, each listing’s object count, page count and sizes, or how far a failed listing got, every New file: and Changed: decision, the keys to delete, the part size for an object too big for the configured one (Using 11.0MB parts for ...), every key copied or deleted, progress every 50 copies, and per-bucket timings.

The summary

======================================================================
FINAL SUMMARY
======================================================================
Started:  2026-11-05 08:10:53
Finished: 2026-11-05 08:10:53
Duration: 0.0s (0s)

Buckets:
  - Processed: 2
  - Created:   1

Objects:
  - Copied:    5
  - Deleted:   0

Data transferred: 843.0KB
Average throughput: 36.6MB/s

✓ Status: COMPLETED SUCCESSFULLY
======================================================================

Processed counts buckets that got as far as the comparison, whatever happened to their copies. A Skipped line appears when a bucket couldn’t be checked, created or listed, Average throughput only when something was copied, and with any errors the status line becomes ⚠ Errors encountered: N followed by Status: COMPLETED WITH ERRORS. A run stopped by Ctrl+C, SIGTERM or a fatal error prints no summary.

Sizes are in powers of 1024 with B, KB, MB, GB and TB labels. Durations under a minute are in seconds, under an hour in minutes to one decimal place (12.5m), and beyond that in hours and minutes (2h 5m).