High Sierra Time Machine Changes...

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High Sierra Time Machine Changes...

Rodney
Has anyone seen anything about changes to Time Machine in High Sierra? As best I can tell, there have been significant changes under the hood.

I had an unresolved issue in Sierra, so I kept a close watch on Time Machine after I upgraded to see if things improved. They did, but I noticed other interesting things in the log.

It seems that the way Time Machine works for APFS filesystems is to first clone the entire filesystem. Because of the way APFS works, this incurs no significant time or space penalty. I can’t verify the space penalty, but the time stamps in the log show that the cloning is fast.

Next, the clone is mounted, and Time Machine then backs up the clone to the regular Time Machine volume. This is a good way to do it, because the clone won’t change during the backup. Any changes made during the backup will be written to the original volume, and will be picked up during the next backup.

After the backup completes, Time Machine dismounts the clone and cleans up after itself.

This sounds good, and should result in more consistent backups. However, I did notice a problem. Shortly after I’d installed High Sierra, I noticed that the clones weren’t dismounting. I discovered that I had four of them mounted. I contacted Apple Support, and while I was chatting with them, a 5th one showed up. They’re supposed to get back to me on this.

After the call, I rebooted, and the mounted clones went away. I still see entries in the log occasionally that show that a dismount failed, but it seems to get cleaned up later.


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Re: High Sierra Time Machine Changes...

@lbutlr
On 02 Oct 2017, at 12:05, Rodney <[hidden email]> wrote:
> Has anyone seen anything about changes to Time Machine in High Sierra? As best I can tell, there have been significant changes under the hood.

I don't think there have. TM is still using HFS+ and other than the additions of snapshots a couple of versions back, there's not been much changed in it for several years.

Changes are coming, that is certain. But when?

> I had an unresolved issue in Sierra, so I kept a close watch on Time Machine after I upgraded to see if things improved. They did, but I noticed other interesting things in the log.
>
> It seems that the way Time Machine works for APFS filesystems is to first clone the entire filesystem. Because of the way APFS works, this incurs no significant time or space penalty. I can’t verify the space penalty, but the time stamps in the log show that the cloning is fast.

That is the snapshot feature. It is basically automatic and use 0 space with APFS, and this sort of thing has been going on on my MBP for a couple of versions (though far less efficiently).

> Next, the clone is mounted, and Time Machine then backs up the clone to the regular Time Machine volume. This is a good way to do it, because the clone won’t change during the backup. Any changes made during the backup will be written to the original volume, and will be picked up during the next backup.

This is all a mechanism of APFS though, and not of TimeMachine.

> After the backup completes, Time Machine dismounts the clone and cleans up after itself.

The clone remains as a snapshot for quite a while. You can see the snapshots with the tmutil command:

$  tmutil listlocalsnapshotdates
Snapshot dates for all disks:
2017-10-01-124454
2017-10-01-134335
2017-10-01-144315
2017-10-01-155130
2017-10-01-174641
2017-10-01-184259
2017-10-01-204157
2017-10-01-214231
2017-10-01-224449
2017-10-01-234340
2017-10-02-004234
2017-10-02-024228
2017-10-02-034241
2017-10-02-044751
2017-10-02-060243
2017-10-02-081135
2017-10-02-093404
2017-10-02-105317
2017-10-02-115155

> This sounds good, and should result in more consistent backups. However, I did notice a problem. Shortly after I’d installed High Sierra, I noticed that the clones weren’t dismounting. I discovered that I had four of them mounted. I contacted Apple Support, and while I was chatting with them, a 5th one showed up. They’re supposed to get back to me on this.

I currently have three mounted, but since they don't hurt anything and take up no space or resources, I am not concerned. On my laptop at one point I had 15. But again, it's 0 overhead. I haven't seen more than three recently, and usually it is only one or zero.

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Re: High Sierra Time Machine Changes...

Rodney

> On Oct 2, 2017, at 20:18, @lbutlr <[hidden email]> wrote:

> That is the snapshot feature. It is basically automatic and use 0 space with APFS, and this sort of thing has been going on on my MBP for a couple of versions (though far less efficiently).

I don’t remember seeing it before High Sierra, but I’ll admit that with Sierra I was mostly concerned about when (or if) the backups ran. I looked for errors, but otherwise I didn’t look at the log details too closely.

> This is all a mechanism of APFS though, and not of TimeMachine.

Snapshots are a feature of APFS, true, but Time Machine is now using them. I’d thought that was new.

> The clone remains as a snapshot for quite a while. You can see the snapshots with the tmutil command:
>
> $  tmutil listlocalsnapshotdates

Thanks for that. I’ll have a look. Maybe, “It ain’t a bug, it’s a feature."

> I currently have three mounted, but since they don't hurt anything and take up no space or resources, I am not concerned. On my laptop at one point I had 15. But again, it's 0 overhead. I haven't seen more than three recently, and usually it is only one or zero.

What concerned me was that the clone wouldn’t always have 0 overhead. I thought that as the base volume changed, then the clone would preserve the original content, so as the two volumes diverged, the clone(s) would take up more and more space.


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