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From: David Empson on 17 May 2010 20:07
TaliesinSoft <taliesinsoft(a)me.com> wrote:

> On 2010-05-17 12:53:14 -0500, Lewis said:
>
> > In message <85dc19Fmj2U3(a)mid.individual.net> TaliesinSoft
> > <taliesinsoft(a)me.com> wrote:
> >> The Time Machine documentation found at
> >> <http://www.apple.com/macosx/what-is-macosx/time-machine.html> states
> >> that "Time Machine saves the hourly backups for the past 24 hours,
> >> daily backups for the past month, and weekly backups for everything
> >> older than a month." Just before posting this a check showed that there
> >> was an available backup "Yesterday at 12:00 PM" and that the most
> >> recent backup was "Today at 12:00 PM". Between these two are 23 more
> >> backups, each on the hour, 1:00 PM, 2:00 PM, 3:00 PM, and so on,
> >> meaning that the most recent 25 hours of backups are saved. This
> >> pattern of there being 25 most recent hourly backups is always true. I
> >> am using TimeMachineEditor to schedule the backups to occur each hour
> >> on the hour. So is the fact that there are 25 most recent hours instead
> >> of 24 because of TimeMachineEditor or is it a mistake in Apple's
> >> documentation?
> >
> > Neither. Apple's documentation is correct. It takes 25 backups to cover
> > 24 hours completely, since you will have (as you saw) a backup at 12pm
> > yesterday and one at 12pm today. 25 backups for 24 hours.
>
> But that first backup, the one at 12 PM yesterday, covered the changes
> made since 11 AM yesterday

No, it covers the changes since 12 PM the previous day, since all the
earlier hourly backups have been deleted.

At 1 PM today, after creating another hourly backup, TM will have hourly
backups going back as far as 1 PM yesterday (25 in total).

At 2 PM today, after creating another hourly backup, TM can delete the 1
PM backup from yesterday. The 2 PM backup from yesterday is now a two
hour backup, followed by 24 hourly backups.

At 3 PM today, the same thing happens - the 3 PM backup from yesterday
is now a three hour backup, followed by 24 hourly backups.

....

After the 11 AM backup, you are left with the 11 AM backup from
yesterday (covering 23 hours), followed by 24 hourly backups.

After the 12 PM backup, you are left with the 12 PM backup from
yesterday (covering 24 hours), followed by 24 hourly backups. The 12 PM
backup from yesterday will be retained as a daily backup.

> thus, as I see it, what I always will have, if I have remained connected,
> are the last 25 consecutive hours of backups.

No, you have 24 hourly backups preceded by either a daily backup or a
partial daily backup (which might happen to only contain an hour of
changes, at 1 PM in your case).

--
David Empson
dempson(a)actrix.gen.nz
From: TaliesinSoft on 18 May 2010 00:12
On 2010-05-17 19:07:40 -0500, David Empson said:

> TaliesinSoft <taliesinsoft(a)me.com> wrote:
>
>> On 2010-05-17 12:53:14 -0500, Lewis said:
>>
>>> In message <85dc19Fmj2U3(a)mid.individual.net> TaliesinSoft
>>> <taliesinsoft(a)me.com> wrote:
>>>> The Time Machine documentation found at
>>>> <http://www.apple.com/macosx/what-is-macosx/time-machine.html> states
>>>> that "Time Machine saves the hourly backups for the past 24 hours,
>>>> daily backups for the past month, and weekly backups for everything
>>>> older than a month." Just before posting this a check showed that there
>>>> was an available backup "Yesterday at 12:00 PM" and that the most
>>>> recent backup was "Today at 12:00 PM". Between these two are 23 more
>>>> backups, each on the hour, 1:00 PM, 2:00 PM, 3:00 PM, and so on,
>>>> meaning that the most recent 25 hours of backups are saved. This
>>>> pattern of there being 25 most recent hourly backups is always true. I
>>>> am using TimeMachineEditor to schedule the backups to occur each hour
>>>> on the hour. So is the fact that there are 25 most recent hours instead
>>>> of 24 because of TimeMachineEditor or is it a mistake in Apple's
>>>> documentation?
>>>
>>> Neither. Apple's documentation is correct. It takes 25 backups to cover
>>> 24 hours completely, since you will have (as you saw) a backup at 12pm
>>> yesterday and one at 12pm today. 25 backups for 24 hours.
>>
>> But that first backup, the one at 12 PM yesterday, covered the changes
>> made since 11 AM yesterday
>
> No, it covers the changes since 12 PM the previous day, since all the
> earlier hourly backups have been deleted.
>
> At 1 PM today, after creating another hourly backup, TM will have hourly
> backups going back as far as 1 PM yesterday (25 in total).
>
> At 2 PM today, after creating another hourly backup, TM can delete the 1
> PM backup from yesterday. The 2 PM backup from yesterday is now a two
> hour backup, followed by 24 hourly backups.
>
> At 3 PM today, the same thing happens - the 3 PM backup from yesterday
> is now a three hour backup, followed by 24 hourly backups.
>
> ...
>
> After the 11 AM backup, you are left with the 11 AM backup from
> yesterday (covering 23 hours), followed by 24 hourly backups.
>
> After the 12 PM backup, you are left with the 12 PM backup from
> yesterday (covering 24 hours), followed by 24 hourly backups. The 12 PM
> backup from yesterday will be retained as a daily backup.
>
>> thus, as I see it, what I always will have, if I have remained connected,
>> are the last 25 consecutive hours of backups.
>
> No, you have 24 hourly backups preceded by either a daily backup or a
> partial daily backup (which might happen to only contain an hour of
> changes, at 1 PM in your case).

Well, as I write this, today's backups run from 12 AM to 10 PM (that's
24 backps) and yesterday's backups have one at 11 PM and one at 12 AM.
That 11 PM backup yesterday is the one that preceded the 12 AM backup
today, so together that makes 25 consecutive backups. The 12 AM backup
yesterday is the one that will become the daily backup for yesterday,
Sunday. So it still seems to me that if my backups do indeed run when
scheduled by TimeMachineEditor, and as long as I remain connected, I
will have 25 consecutive hourly backups ending with the most recent,
and not the 24 that the Apple documentation states.

--
James Leo Ryan
Austin, Texas

From: TaliesinSoft on 18 May 2010 00:32
On 2010-05-17 23:12:38 -0500, TaliesinSoft said:

[in my immediately preceding posting in this thread]

> Well, as I write this, today's backups run from 12 AM to 10 PM (that's
> 24 backps) and yesterday's backups have one at 11 PM and one at 12 AM.
> That 11 PM backup yesterday is the one that preceded the 12 AM backup
> today, so together that makes 25 consecutive backups. The 12 AM backup
> yesterday is the one that will become the daily backup for yesterday,
> Sunday. So it still seems to me that if my backups do indeed run when
> scheduled by TimeMachineEditor, and as long as I remain connected, I
> will have 25 consecutive hourly backups ending with the most recent,
> and not the 24 that the Apple documentation states.

Oops, that first sentence should have stated "today's backups run from
12 AM to 11 PM (that's 24 backups)...." A backup, one I hadn't noticed
was running while I was composing my response, and that brought about
the change from 10 PM to 11 PM.

--
James Leo Ryan
Austin, Texas

From: MartinC on 18 May 2010 03:06
TaliesinSoft wrote:

> But that first backup, the one at 12 PM yesterday, covered the changes
> made since 11 AM yesterday, thus, as I see it, what I always will have,
> if I have remained connected, are the last 25 consecutive hours of
> backups.

Referring to your original post:

This is exactly your mistake... the oldest backup (12 PM yesterday, 12:00 in
24h mode) would only be an "hourly" backup if the one from 11 AM (11:00 in
24h mode) still exists. But it doesn't, so it actually is the latest daily
backup, because it covers all changes "since the day before", not "since the
hour before".

Like this:

"some backup from the day before yesterday" (second last daily backup)
yesterday 12 PM / 12:00 (last daily backup)
yesterday 1 PM / 13:00 (1. hourly backup)
yesterday 2 PM / 14:00 (2. hourly backup)
yesterday 3 PM / 15:00 (3. hourly backup)
....
yesterday 11 PM / 23:00 (11. hourly backup)
midnight / 0:00 (12. hourly backup)
today 1 AM / 1:00 (13. hourly backup)
....
today 11 AM / 11:00 (23. hourly backup)
today 12 PM / 12:00 (24. hourly backup) Bingo! ;-)

From: Richard Maine on 18 May 2010 03:35
TaliesinSoft <taliesinsoft(a)me.com> wrote:

> I will have 25 consecutive hourly backups ending with the most recent, and
> not the 24 that the Apple documentation states.

As long as one is being that picky (and I'm not at all sure why you
care, but I'll play along for one post at any rate), one should quote
the documentation accurately. If you misquote it, then you don't get any
credit towards the game of picking nits.

If your initial quote was accurate, it said nothing about "24 backups".
It did say "for the past 24 hours", but that's not the same thing. It
takes 25 hourly backups to completely cover 24 hours - that is to be
able to restore to a time 24 hours ago. Right after you have made a
backup, if you had only 24 hourly backups, the oldest would be from 23
hours ago. Note that a backup has no information about covering some
period of time; that backup from 23 hours ago doesn't somehow count as
covering the 24'th hour because it was an hourly backup and thus has
updates from the previous hour. What it has is a backup from one point
in time, and that point was 23 hours ago.

If you want to argue with that... well... I think you'd be wrong, but
regardless of that, misquoting looses you "points". To play the
nitpicking game, you would need to quote the documentation accurately
and then explain why you think it is equivalent to saying 24 backups.

Forgetting that it takes n+1 points to bracket an interval that has a
width of n times the point separation is actually a quite common basic
programming error, often resulting in such things as exceeding array
bounds.

--
Richard Maine | Good judgment comes from experience;
email: last name at domain . net | experience comes from bad judgment.
domain: summertriangle | -- Mark Twain
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