{"id":24534,"date":"2019-03-07T15:59:20","date_gmt":"2019-03-07T20:59:20","guid":{"rendered":"https:\/\/mjtsai.com\/blog\/?p=24534"},"modified":"2019-03-07T15:59:20","modified_gmt":"2019-03-07T20:59:20","slug":"how-time-machine-makes-backups-with-apfs","status":"publish","type":"post","link":"https:\/\/mjtsai.com\/blog\/2019\/03\/07\/how-time-machine-makes-backups-with-apfs\/","title":{"rendered":"How Time Machine Makes Backups With APFS"},"content":{"rendered":"

Howard Oakley<\/a>:<\/p>\n

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The preparatory sequence identifies and deletes expired local snapshots. According to Apple’s Support Note<\/a>, these local snapshots are kept for 24 hours; although the log entries below indicate a shorter period, later backups confirm that this is normally the case, and you should expect to find a full 24 hours of snapshots at any time.<\/p>\n

backupd<\/code> then copies changed items to the backup destination. In order to maintain the impression that each backup is a complete copy of the source volume, it then makes hard links to all the unchanged files and folders. It is able to do this as, unlike many file systems, HFS+ supports directory hard links as well as those to files.<\/p>\n

[…]<\/p>\n

backupd<\/code> then checks that there is sufficient free space on the backup destination, and if there is, performs that same process as with HFS+, copying changed items and making hard links to the rest. That is followed by new steps, which save a clone family cache<\/em> to the new backup folder, and back-up-later caches<\/em> there too. The precise purpose of these isn’t yet clear, although the latter may well list files which changed as the backup was being made.<\/p>\n

[…]<\/p>\n

I have been writing that Time Machine has fallen behind macOS, at least in respect of its reliance on the HFS+ file system for backups, which results from its use of directory hard links. This implementation of Time Machine for APFS is perhaps best viewed as version 1.5: it now takes best advantage of the new file system as its source, but has yet to find a new backup method and format appropriate to an APFS backup destination.<\/p>\n<\/blockquote>\n\n

Howard Oakley<\/a>:<\/p>\n

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That old [HFS+] system periodically failed, perhaps when FSEvents lost track of recent changes, or became corrupted. In those circumstances, Time Machine would perform a lengthy ‘deep traversal scan’ to determine what needed to be backed up, which could in some cases take several hours. One strong case for adopting a new approach with APFS was to reduce the frequency of those deep traversals by using a more robust mechanism for determining what to back up. There’s no evidence that the new snapshot-based system is any quicker – indeed, in many cases it may perform more slowly than using FSEvents.<\/p>\n

[…]<\/p>\n

However, deep traversal scans do still occur on APFS volumes. In one period of only 8 hours, my iMac Pro undertook and completed two such scans, as shown in the T2M2 report below.<\/p>\n<\/blockquote>\n\n

Previously:<\/p>\n