rsync difference between --size-only and --ignore-times

Question

I m trying to understand what the difference is between two options   rsync --size-only   and  rsync --ignore-times   It is my understanding that by default rsync will compare both the timestamps and the file sizes in order to decide whether or not a file should be synchronized or not  The options above allow the user to influence this behavior   Both options seem  at least verbally to result in the same thing  comparing by size only   Am I missing something subtle here

User · Answer

You are missing that rsync can also compare files by checksum.

--size-only means that rsync will skip files that match in size, even if the timestamps differ. This means it will synchronise fewer files than the default behaviour. It will miss any file with changes that don't affect the overall file size. If you have something that changes the dates on files without changing the files, and you don't want rsync to spend lots of time checksumming those files to discover they haven't changed, this is the option to use.

--ignore-times means that rsync will checksum every file, even if the timestamps and file sizes match. This means it will synchronise more files than the default behaviour. It will include changes to files even where the file size is the same and the modification date/time has been reset to the original value. Checksumming every file means it has to be entirely read from disk, which may be slow. Some build pipelines will reset timestamps to a specific date (like 1970-01-01) to ensure that the final build file is reproducible bit for bit, e.g. when packed into a tar file that saves the timestamps.

User · Answer

On a Scientific Linux 6 7 system  the man page on rsync says   --ignore-times          don t skip files that match size and time   I have two files with identical contents  but with different creation dates    root windstorm     ls -ls  tmp master usercron  tmp new usercron 4 -rwxrwx--- 1 root root 1595 Feb 15 03 45  tmp master usercron 4 -rwxrwx--- 1 root root 1595 Feb 16 04 52  tmp new usercron   root windstorm     diff  tmp master usercron  tmp new usercron  root windstorm     md5sum  tmp master usercron  tmp new usercron 368165347b09204ce25e2fa0f61f3bbd   tmp master usercron 368165347b09204ce25e2fa0f61f3bbd   tmp new usercron   With --size-only  the two files are regarded the same    root windstorm     rsync -v --size-only -n   tmp new usercron  tmp master usercron  sent 29 bytes  received 12 bytes  82 00 bytes sec total size is 1595  speedup is 38 90  DRY RUN    With --ignore-times  the two files are regarded different    root windstorm     rsync -v --ignore-times -n   tmp new usercron  tmp master usercron usercron  sent 32 bytes  received 15 bytes  94 00 bytes sec total size is 1595  speedup is 33 94  DRY RUN    So it does not looks like --ignore-times has any effect at all

User · Answer

There are several ways rsync compares files -- the authoritative source is the rsync algorithm description  https   www andrew cmu edu course 15-749 READINGS required cas tridgell96 pdf  The wikipedia article on rsync is also very good   For local files  rsync compares metadata and if it looks like it doesn t need to copy the file because size and timestamp match between source and destination it doesn t look further  If they don t match  it cp s the file  However  what if the metadata do match but files aren t actually the same  Then rsync probably didn t do what you intended   Files that are the same size may still have changed  One simple example is a text file where you correct a typo -- like changing  teh  to  the   The file size is the same  but the corrected file will have a newer timestamp  --size-only says  don t look at the time  if size matches assume files match   which would be the wrong choice in this case   On the other hand  suppose you accidentally did a big cp -r A B yesterday  but you forgot to preserve the time stamps  and now you want to do the operation in reverse rsync B A  All those files you cp ed have yesterday s time stamp  even though they weren t really modified yesterday  and rsync will by default end up copying all those files  and updating the timestamp to yesterday too  --size-only may be your friend in this case  modulo the example above    --ignore-times says to compare the files regardless of whether the files have the same modify time  Consider the typo example above  but then not only did you correct the typo but you used touch to make the corrected file have the same modify time as the original file -- let s just say you re sneaky that way  Well --ignore-times will do a diff of the files even though the size and time match

User · Answer

The short answer is that --ignore-times does more than its name implies  It ignores both the time and size  In contrast  --size-only does exactly what it says     The long answer is that rsync has three ways to decide if a file is outdated    Compare the size of source and destination  Compare the timestamp of source and destination  Compare the static checksum of source and destination    These checks are performed before transferring data  Notably  this means the static checksum is distinct from the stream checksum - the later is computed while transferring data   By default  rsync uses only 1 and 2  Both 1 and 2 can be acquired together by a single stat  whereas 3 requires reading the entire file  this is independent from reading the file for transfer   Assuming only one modifier is specified  that means the following    By using --size-only  only 1 is performed - timestamps and checksum are ignored  A file is copied unless its size is identical on both ends  By using --ignore-times  neither of 1  2 or 3 is performed  A file is always copied  By using --checksum  3 is used in addition to 1  but 2 is not performed  A file is copied unless size and checksum match  The checksum is only computed if size matches

[rsync] rsync: difference between --size-only and --ignore-times

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