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Apple’s Slow Time Machine

 
 
Lawrence D'Oliveiro
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      10-04-2010
In message <i8apff$uht$(E-Mail Removed)-september.org>, victor wrote:

> On 3/10/2010 11:34 p.m., Lawrence D'Oliveiro wrote:
>
>> In message<i82qih$nll$(E-Mail Removed)-september.org>, victor wrote:
>>
>>> Time machine works in the background for home users.

>>
>> Not very well, judging from the original article.

>
> Experienced Mac users on this group say it works well.


Maybe it’s because they don’t know any better. They don’t realize that, if
you have 100GB of data of which only a few gigs has changed since the last
backup, it should only take a few minutes to find and copy those changes;
they’re accustomed to it taking hours to complete.
 
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victor
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      10-04-2010
On 4/10/2010 3:00 p.m., Lawrence D'Oliveiro wrote:
> In message<i8apff$uht$(E-Mail Removed)-september.org>, victor wrote:
>
>> On 3/10/2010 11:34 p.m., Lawrence D'Oliveiro wrote:
>>
>>> In message<i82qih$nll$(E-Mail Removed)-september.org>, victor wrote:
>>>
>>>> Time machine works in the background for home users.
>>>
>>> Not very well, judging from the original article.

>>
>> Experienced Mac users on this group say it works well.

>
> Maybe it’s because they don’t know any better.


Maybe it works well.
 
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Lawrence D'Oliveiro
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      10-04-2010
In message <i8bcvs$426$(E-Mail Removed)-september.org>, victor wrote:

> Maybe it works well.


Maybe it’s because they don’t know any better. They don’t realize that, if
you have 100GB of data of which only a few gigs has changed since the last
backup, it should only take a few minutes to find and copy those changes;
they’re accustomed to it taking hours to complete.
 
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David Empson
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      10-04-2010
Lawrence D'Oliveiro <(E-Mail Removed)_zealand> wrote:

> In message <i8apff$uht$(E-Mail Removed)-september.org>, victor wrote:
>
> > On 3/10/2010 11:34 p.m., Lawrence D'Oliveiro wrote:
> >
> >> In message<i82qih$nll$(E-Mail Removed)-september.org>, victor wrote:
> >>
> >>> Time machine works in the background for home users.
> >>
> >> Not very well, judging from the original article.

> >
> > Experienced Mac users on this group say it works well.

>
> Maybe it's because they don't know any better. They don't realize that, if
> you have 100GB of data of which only a few gigs has changed since the last
> backup, it should only take a few minutes to find and copy those changes;
> they're accustomed to it taking hours to complete.


Um, what?

The only time I've seen Time Machine taking "hours" to complete a backup
is when it is doing its initial backup, on a computer with a large
number of files, so it is having to copy everything.

I use Time Machine to backup up my laptop to an external Firewire hard
drive. It typically takes less than two minutes to back up everything
which changed since the last backup, which is a tiny proportion of the
total number of files on the source volume. Total size of files on my
source volume is about 250 GB, and the total file count is in the order
of several hundred thousand.

If I've done something like install a system update, the next backup
takes somewhat longer (because a lot more files changed), but still only
a few minutes. My first backup today (catching up from Thursday) took
less than 5 minutes to back up 3 GB, and another 2 minutes to purge
several hourly backups from last week. The next hourly backup took less
than a minute (very little had changed).

I don't even notice it doing a backup unless I listen to the hard drive
or see the spinning Time Machine indicator in the menu bar.

--
David Empson
http://www.velocityreviews.com/forums/(E-Mail Removed)
 
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Lawrence D'Oliveiro
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      10-05-2010
In message <i81qfe$lv4$(E-Mail Removed)>, Another Me wrote:

> The majority of the Photoshop users will be businesses, and they won't be
> using time machine as a backup.


What will they be using, pray tell?

> Now for the people who have purchased a lot of digital downloaded movies,
> this will only be backed up by Time Machine the once, it generally doesn't
> change so won't need backed up again.


That’s where the problem comes in. The way Time Machine avoids making
multiple copies of unchanged files is by putting in hard links to the
existing copies.

This making and breaking of hard links is a common operation on Unix/Linux
systems, so most Unix/Linux filesystems are designed to ensure it’s cheap to
perform. Not, however, the HFS+ filesystem which is Apple’s preferred
filesystem under MacOS. That’s where the performance problems with Time
Machine come from.

Read the article
<http://www.zdnet.com/blog/storage/why-time-machine-is-broken/1097> for more
info on this.
 
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Lawrence D'Oliveiro
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      10-07-2010
In message <1jpufdf.1hfyp181eb0telN%(E-Mail Removed)>, David Empson
wrote:

> I don't even notice it doing a backup unless I listen to the hard drive
> or see the spinning Time Machine indicator in the menu bar.


Sounds like you’re not doing much else with the hard drive at the same time.

The problem arises because it has to lock various entire filesystem B-trees
while creating and inserting hard links. On its own this may not matter
much, but if other processes are performing concurrent operations on the
filesystem, they will be blocked by these locks. Or conversely, if they get
in first, they will hold up the Time Machine processes.

More details here
<http://www.zdnet.com/blog/storage/why-time-machine-is-broken/1097>.
 
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David Empson
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      10-08-2010
Lawrence D'Oliveiro <(E-Mail Removed)_zealand> wrote:

> In message <1jpufdf.1hfyp181eb0telN%(E-Mail Removed)>, David Empson
> wrote:
>
> > I don't even notice it doing a backup unless I listen to the hard drive
> > or see the spinning Time Machine indicator in the menu bar.

>
> Sounds like you're not doing much else with the hard drive at the same time.


Normal use patterns. I can't tell that a backup is occurring, even if
I'm doing something with a moderate amount of disk activity.

Something needing heavy disk activity will be impacted due to the
overhead of Time Machine copying data, etc., but Time Machine's backupd
runs at lower priority than normal to mitigate this somewhat.

If you are doing something important and time critical with lots of disk
I/O, you should turn off Time Machine during that task. If that is a
regular requirement then you should be looking at a different backup
solution, or using Time Machine with some kind of scheduling tool.

> The problem arises because it has to lock various entire filesystem B-trees
> while creating and inserting hard links.


On the backup drive? So what? I'm only using that drive for backups, so
a file system lock will only delay the backup. I don't notice any impact
on file I/O on the main drive (apart from data copying).

It is not a good idea to use the Time Machine backup volume for other
things, because Time Machine is designed to (eventually) completely fill
the volume, which won't leave you space for other files.

You can manipulate this behaviour by partitioning a drive or setting up
disk images to reserve space for Time Machine or other uses, in which
case I expect any file system locking is only going to affect the
partition or disk image in which Time Machine is operating.

> On its own this may not matter much, but if other processes are performing
> concurrent operations on the filesystem, they will be blocked by these
> locks. Or conversely, if they get in first, they will hold up the Time
> Machine processes.
>
> More details here
> <http://www.zdnet.com/blog/storage/why-time-machine-is-broken/1097>.


Yes, I read it already. Most of it doesn't seem to agree with my
observed reality, nor that of anyone else I know who is using Time
Machine.

The only signficant point I agree with is that Time Machine doesn't cope
well with multi-gigabyte files that have small changes, as it needs to
back up the entire file again.

There are ways around that problem as well: things like Entourage
databases (single file containing all of your e-mail) and virtual hard
drives for VMs can be set up inside a sparse bundle disk image, which is
implemented as a folder containing lot of "band" files of a few
megabytes each. A localised change will only affect a few of the bands,
so only the changed parts need to be backed up again.

--
David Empson
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