Time Machine: Which oldest backups are deleted?

I have looked in vain for an unambiguous answer to this question.
Hopefully someone here knows the answer from experience (or any other
means).

When Time Machine runs out of space on its disk, it deletes the oldest
backups to make room for new ones. But what if more than one disk (or
computer) is being backed up to the same disk? Will it delete the oldest
backups for the disk it is trying to back up? Or will it instead delete
the oldest backups of any disk it is backing up?

In my searches of available information, I have found hints at both
answers but no definite information.

This is important to me because of the following anticipated scenario:

After backing up N disks for awhile, Time Machine has filled its disk
and is routinely removing old backups to make room for new ones. Now, I
add a new disk to be backed up. Since that disk currently has no backups
on the disk, Time Machine can make room only by deleting the oldest
backups of one or more other disks. Will it do that, or will I need to
get another disk?

I am hoping to start using Time Machine soon. But I need to answer this
question so I can decide how many disks (and of what sizes) I need to
purchase.

/Ed

In article <yZednZIL4P3zyF7anZ2dnUVZ_oOnnZ2d.TakeThisOut@comcast.com>,
Ed Anson <EdAnson.TakeThisOut@comcast.net> wrote:

> I have looked in vain for an unambiguous answer to this question.
> Hopefully someone here knows the answer from experience (or any other
> means).

You won't get an unambiguous answer because your question makes an
incorrect assumption about how Time Machine works.

> When Time Machine runs out of space on its disk, it deletes the oldest
> backups to make room for new ones.

No. It deletes some of the individual changed files it's keeping. It
works on the level of individual files, not entire backups. There are
some exceptions to that because full backups are taken monthly.

> But what if more than one disk (or
> computer) is being backed up to the same disk? Will it delete the oldest
> backups for the disk it is trying to back up? Or will it instead delete
> the oldest backups of any disk it is backing up?

Each backup assumes that it is the only user of the disk, and once the
disk is full each backup is handled as if it has reached its maximum
allowed size (slight hand-waving since freeing space in one backup may
allow another backup to expand).

> In my searches of available information, I have found hints at both
> answers but no definite information.
>
> This is important to me because of the following anticipated scenario:
>
> After backing up N disks for awhile, Time Machine has filled its disk
> and is routinely removing old backups to make room for new ones. Now, I
> add a new disk to be backed up. Since that disk currently has no backups
> on the disk, Time Machine can make room only by deleting the oldest
> backups of one or more other disks. Will it do that, or will I need to
> get another disk?

You'll need to free some space yourself on the disk since the first
backup of the new volume will just see that there's too little space
available to take a useful backup.

One solution for the situation you describe is to partition your backup
destination volume into separate volumes for each disk you intend to
backup. That way you can arrange how much space you want to allocate to
backing up each of your disks. When you add a new disk you'll have the
interesting task of deciding which backups should be slimmed down or
deleted to make way.

Simon.