rsync completed in both directions, but size of directories is different. How it's possible?

Question

I want to sync 2 directories (/src & /dst) to mirror all the files in both of them.

Here is a steps:

1. sudo rsync -vaP --stats /src /dst -> completed without errors
2. sudo rsync -vaP --stats /dst /src -> completed without errors
3. diff -rq /src /dst-> doesn't show any diffs.
4. du -s /src && du -s /dst shows different sizes (10% diff).

How it's possible? I'm completely stuck with this dilemma.

Answer 1

• Sparse files may be expanded on copy when the -S flag is not used. (Will make the destination take more space)
• Hard links within the tree may be expanded to separate files on copy when the -H flag is not used. (Will make the destination take more space)
• Filesystems may have different allocation sizes. A one-byte file may take up 512 bytes of disk allocation on one filesystem and may take up 4096 bytes (or even more) on another. If your tree has a lot of small files, this will make a large difference. (Destination may take more or less space depending on the particulars)
• Directories may be much larger than necessary to hold the current contents on some filesystems. When the contents are copied, the directory will be smaller on the destination. Not normally a big deal, but some pathological directories can be enormous. (Will make the destination take less space)

It's also possible for filesystems to have different compression/deduplication/redundancy settings leading to different storage requirements for the data. But that's less common, and even when present the differences aren't always visible via du.