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As I understand it Time Machine will create the folder structure and create a copy of all of the files that are to be included in the Time Machine backup during the first backup. After that a hard link is created during every subsequent backup for files that have not been changed.

The question that I have is how files that have been changed since the last backup are backed up in the subsequent backup. I thought that it copied the entire modified file to the TM backup, but it appears that that isn't how TM works and I was hoping for some clarification.

During a recent practice test that I took for ACSP 10.12 it asked if, for a 5MB file if it was modified such that the new version was 6MB, would the TM have a separate 5MB and 6MB file or would it backup the 1MB difference between the two files and just retain the 1MB addition in the most recent TM backup. I figured that, though it may take up additional space to have a separate copy of the 5MB version and the 6MB version, it would provide a safety net in the event that an early version were deleted or corrupted - effectively ruining every subsequent version of that file. But the practice test said that it just saves the 1MB difference.

I was just looking for some clarification to help improve my understanding. Thanks in advance for any help!

  • If Time Machine culls the oldest backups when the TM volume fills up, wouldn't that destroy the original version that the 1MB increment would be added onto? – QMord Nov 27 '17 at 3:20
  • Files are not deleted when hard links still point to it. – Wilbert Nov 27 '17 at 11:51
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After the initial Time Machine backup of the entire volume, sans exclusions, Time Machine always backs up the entire file of each file that has changed or was created since the last backup, not just the changed part.

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    Based on experience this sounds right–whenever one of my VirtualBox storage volume changes, based on files sizes, the entire file appears to be backed up again–but it seems to contradict what @QMord said was on his ACSP test–which is an official source. Do you have a source, or is this anecdotal? Perhaps Time Machine uses some hybrid method? – JMY1000 Nov 27 '17 at 3:37
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    @JMY1000, The original source is now offline but available through the Wayback Machine: How Time Machine Works its Magic I also have throughly examined the file structure of a Time Machine backup over time and know for a fact that it backs up an entire file that has changed not just what changed in the file. – user3439894 Nov 27 '17 at 3:48
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    I can confirm that this answer is correct. Apple does make changes from time to time, such as how local snapshots are dealt with in macOS High Sierra, but Time Machine has always made a full backup of any file that has changed since the last backup. So, assuming its default behaviour, if you've been working on a single document for 12 of the last 24 hours, then you will have 12 (probably 13 actually) full backups of that single document. – Monomeeth Nov 27 '17 at 4:04
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    MacOS helps Time Machine by recording what happens in the file system. – Thorbjørn Ravn Andersen Nov 27 '17 at 12:28
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    @JMY1000 When I used VMWare Player for virtual machines years back, I created the VM's on a striped volume (where the virtual disk is split into a myriad of small files, where only those that were affected by file changes needed to be backed up). This did a reasonable job of keeping the changes small for Time Machine. You will most likely like to keep swap space in the VM to a minimum. – Thorbjørn Ravn Andersen Nov 27 '17 at 12:30

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