Re: Time Machine and Deleting Files

In article <siegman-39E020.09011207102009@xxxxxxxxxxxxxxxxx>,
AES <siegman@xxxxxxxxxxxx> wrote:

It only does that if there's not enough room for the new stuff, which is
what he meant by "until the backup drive gets full".

That's completely wrong. Time Machine deletes backups all the time. The
thinning process eliminates recent hourly and daily backups.

Time Machine only does . . . . . . . . . . . .


Is there a place where **Apple** itself (rather than squabbling posters)
tries to explain more or less exactly and in some detail what Time
Machine actually does?

Seems to me that would be useful:

a) For informing users as to what Time Machine is doing with their
allegedly "backed up" data; and

b) For letting clever enough Time Machine users respond to Apple with
suggestions for how Apple's implementation might be improved.

I have been doing some research since being told "That's wrong" in
response to my earlier about Time machine being an incremental backup.
My research included the Apple web site.

The descriptions of Time Machine on the Apple web site say that it saves
hourly backups for a day, then daily backups for a week, then weekly
backups until the drive gets full. It does not describe in detail the
process by which the hourly backups are combined into daily and the
daily into weekly.

However, I have done some software design, and I give great credit to
the intelligence of the software designers at Apple.

First, realize that each "backup" is not in fact a complete backup of
all the files in your home folder. That complete backup is made when you
first set up a new storage disk as the target for Time Machine backups.

Each subsequent backup stores only the files that are new or have
changed since the previous backup. The new "backup" stores links to the
older files that were not changed, not the files themselves.

When the hourly backups are consolidated into a daily backup, it would
make sense to me if the daily backup stored all the files that were
created or changed during that day, as well as the links to the files
that had not changed.

Likewise, when the daily backups are consolidated into a weekly backup,
it would make sense if the files that were created or changed during the
week were stored in the weekly backup, along with links to the files
that were not changed.

With this scheme, the consolidation of hourly into daily backups, and
then daily into weekly backups would not result in deletion of any
files. It would result in deletion of all the overhead of links to old
files and so on that are stored in each backup.

The deletion of all that overhead can save a lot of space, without
actually deleting files.

This way, no files are actually deleted when the hourly backups are
consolidated into daily, and the dailies into weekly.

When the backup disk gets full, then Time Machine contemplates deleting
old files. It is set by default to warn the user before is deletes old
files.

As I said above, I have not read a clear description of exactly how Time
Machine consolidates backups, but the scheme described above would make
sense to me, and I would think the designers at Apple are smart enough
to do it that way.

It would be fairly simple to test this. Create a file, make a Time
Machine backup, then delete the file. Tomorrow, check to see if the
daily backup includes that file.

Do the same to test the weekly backup.

I aim to do this test. I will report the results.
.

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