Time Machine – when it goes wrong and how to fix it

Posted on July 9, 2010 by alan

Unfortunately only fixing Mac OS X backup, not the Tardis 🙁 … but, nonetheless, critical.

What bit of software do you really need to be reliable? If anything else goes really wrong you have the backup — but if the backup fails you really are lost.

And Mac OS X Time Machine, while it does have a very pretty interface, is inclined to get stuck sometimes.

This is my own story of how it goes wrong … and how to put it right.

… and throughout I’ve dropped in a few lessons for anyone implementing critical system software — maybe the odd Apple engineer is reading

how to tell when things are wrong

Occasionally Time Machine seems to be stuck, but isn’t really. When you first do a backup, or when you haven’t backed up to a particular disk for ages (perhaps if you have been away on a trip), it can spend several hours ‘preparing’. You can tell it is ‘preparing’ because when you open the Time Machine preferences there is the little barbers pole saying ‘preparing’ 😉

This is when it is running over the disk working out what it needs to backup, and always seems to be the lengthiest operation, actually backing up the disk is often quite fast, and yet, for some reason there is no indication of how far through the ‘preparing’ process it has got.

Lesson 1: make sure you include progress indicators for anything that can take a while, not just the obvious ‘slow’ things.

So, when you see ‘preparing’, just be patient!

However, at least half-a-dozen times over the last year, my Time Machine has got completely stuck. I have seen this happen in three ways:

(i) it is still saying ‘preparing’ after leaving it overnight!

(ii) it starts to transfer to disk, but then gets stuck part way:

(iii) if you look in the Time Machine preferences it says the backup has failed

This last time in fact the first sign was (iii), but it doesn’t actually tell you (if you don’t look) until it has failed for ten days, by which time I was travelling. In the days before Time Machine I always did a manual backup before travelling as I knew that was when things were most likely to go wrong, but now-a-days I have got used to relying on it and forget to check it is working OK … so if you are paranoid about your data, do peek occasionally at Time Machine to check it is still working!

When I got home and told Time Machine to backup to the Time Capsule here rather than my office disk (why can’t it remember that I have two backup disks??). Then (after being very very patient while to was ‘preparing’ for four hours), I saw it got stuck in step (ii) at 1.4 GB or 4.2 GB. Of course progress indicators are never very good for very slow operations, when transferring several GB of data there may be several minutes before the bar even moves a pixel … but I was very very patient and it definitely did not move!

Lesson 2: for very long processes supplement the progress indicator with some other indicator to show things are still working, in this case perhaps amount transferred in last minute

At this point I did the normal things, turn Time Machine On/Off, restart machine a couple of times, etc., but when it persists then you know something is deeply wrong.

so why does it go wrong?

In fact Fiona@lovefibre has found Time Machine flawless for her desktop machine backing up to exactly the same Time Capsule. I am guessing the problem I have is because I use a laptop so possible reasons:

it may go to sleep occasionally, breaking connection to the Time Capsule
maybe the WiFi aerial on a laptop is not as good as the desktop

However, if every laptop failed as often surely Apple would have fixed it by now. So guessing there is an additional factor:

my disk has 196 Gb of data, much of it in smaller document files (word docs, code files, etc.), not just a few giant movies.

The software will be designed to withstand a certain amount of external failure, especially when connecting to disks over WiFi as the Time Capsule is designed to do. However, I imagine that there are places in the code where there are race conditions, or critical portions where external failure really makes a difference. If the external connections are reliable and the backup is quite fast the likelihood of hitting one of the nasty spots in the code is low. However, if you have a lot of data to check and then transfer and the external failures more frequent, then the likelihood of hitting one increases and things start to go wrong.

I see similar problems with other software, Dreamweaver in particular, which has got better, but still can crash if the Internet connection is poor (see also “Why software need never hang“). What happens is that during testing, the test machines often have minimal data, little software (maybe just the operating system and what is being tested), and operate in perfect situations. In such circumstances these hidden flaws never become apparent.

Lesson 3: make sure your test machine is fully loaded with data and applications, and operates in an unreliable environment, so that testing is realistic

However, this is not like Word crashing and losing your most recent edits to one document. When Time Machine fails it seems to occasionally leave something corrupt in the backup disk so that subsequent attempts to backup also fail. There is no excuse for this, the techniques for dealing with potential disk-writing failures are well established in both databases and low-level disk management. For example, one can save a timestamp file at the end of successful operations so that, when returning to the data, if the timestamp file is not there the software knows something went wrong last time.

Maybe Time Machine is trying to be too clever, picking up where it left off when, for example, connection to the disk is broken. If so it clearly needs some additional mechanism to notice “I’ve tried this several times and it keeps going wrong, maybe I need to back off to the last successful state”. Perhaps not something to worry about in less critical software, but not difficult to get right when it is really needed … as in backups!

Lesson 4: build critical software defensively in layers so that errors in one part do not affect the whole; and if saving to disk ensure there is some sort of atomic transaction

The aim during testing should be what I call “fail-fast programming” trying to make sure that failures happen during testing not real use!

One thing I found particularly disturbing about my most recent Time Machine hang is that when I looked at the system console it had regular spats of “unknown SIGSEGV” several times a minute … in the kernel! If you don’t know UNIX internals the ‘kernel’ is the heart of the operating system of the Mac, where all the lowest level work is done and where if something goes wrong everything fails. SIGSEGV means that some bit of software is trying to access a memory location that doesn’t exist. In fact while this is caught it is not so bad, the greater worry is that if it is trying to access non-existent memory, then it may corrupt other memory … and the kernel has access to everything – not good.

Please, please Apple if you cannot get Time Machine to work properly, do not let it affect the kernel!

how to put it right

One might hope that even if Time Machine cannot notice itself there is something wrong at least there would be an option to say “restart yourself”. One might hope, but there is not. However, you can do it yourself by digging a little into the backup disk itself.

First problem is to stop the Time Machine backup if it has hung.

In the Time Machine control panel, you can simply slide the OFF-ON button to OFF. The status should change to ‘stopping’ and after a while stop. Then you can restart the machine and try to fix things.

This is the ideal thing to do, but I find that when Time Machine is really hung this rarely works. I do turn it to OFF, but either it never changes to ‘stopping’ and stays ‘preparing’, or it changes to ‘stopping’, but never does. If this happens the system restart typically doesn’t restart the system as Time Machine won’t stop running. Then, always with much trepidation, I reach for the on/off button on the Mac itself :-/

After doing a hard on/off like this, I usually do anther restart from the Apple menu … not sure if this is necessary, but just to be on the safe side!

Occasionally I skip to the next step before the hard restart.

Then you can start to fix the problem properly.

Find the backup disk. If it is not obvious in the Finder use the ‘Go’ menu and select “Computer”; it shows all the locally connected disks (or it may simply appear in the left hand favourites pane in each Finder window).

If you skipped the restart stage (or of you just peek now to see what it is like when it hasn’t gone wrong), you will see something like “Backup of Alan Dix’s MacBook Pro” (obviously for you it will not be “Alan Dix’s MacBook Pro”!). This is the Time Machine backup. However, if you have restarted the machine with Time Machine off you will have to find the actual disk that you chose as your backup disk and on it look for a file called something like “Alan Dix’s MacBook Pro_0039fc56f8a2.sparsebundle”. This is some form of compressed disk image. In the older versions of Time Machine there was simply a folder with all the backups in it — I felt much more secure. Now this is a single opaque file and I worry that if one day it gets corrupted :-/

Having found the ‘sparsebundle’ double click it and it will display a little pop-up window that says ‘checking volumes’. I keep meaning to see if this ever stops, but I am not patient enough and press the button that says to skip this state and then (after a while) it mounts the disk image and the disk “Backup of Alan Dix’s MacBook Pro” appears.

Double click “Backup of Alan Dix’s MacBook Pro” and look inside and then inside the folder “Backups_backupd” and you find loads of dated folders, which are the actual backups of your system that you can browse if you prefer instead of using the Time Machine interface. In addition there may be one file ending “.inProgress”, which is some sort of internal file created while it is in the middle of doing the backup.

Delete the “.inProgress” file.

In addition, I usually delete the last of the dated folders (sort by “Date Modified” to get the last one). However, if you don’t want to lose the last backup you can try just deleting the “inProgress” file and only delete the last dated backup if Time Machine still gets stuck.

Important: only delete the latest of the dated backup folders (e.g. “2010-06-09-225547” in the screen shot above), NOT the entire “Alan Dix’s Macbook Pro” folder. If you do that you lose all your backups!

I recall doing this all with extreme trepidation the first time, but had got to the point when I couldn’t do backups or access them anyway so had nothing to lose. Actually it seems pretty OK getting in here and doing this sort of thing, the nice thing about Time Machine is that it uses ordinary folder structures that you can peek around in and see are there all secure. I am much happier with this than the kind of backup where you only know if it is working the day you try to restore something! At least half the times I have used such backups over the years I’ve found the backup is in some way corrupt or incomplete. So actually one up for Time Machine 🙂

Now reboot again (for luck). Turn Time Machine back on in the control panel and wait … a long time … it will start ‘preparing’ as if for the first backup … and several hours later hopefully all will be well.

But do remember to set the power save options not to go to sleep in the middle!

In fact the above has always worked for me except for this last time when, for some reason (maybe I missed something on the way?), it hung again and I had to go through the whole process again. This time I waited until yesterday evening before turning Time Machine back on so that I could leave it to do the long 4 hour ‘preparing’ stage without me doing anything else.

And then:

Joy!

PHP syntax checker updated

Posted on June 9, 2010 by alan

Took a quick break today from Project Phoenix¹.

I’ve had a PHP syntax checker on meandeviation for several years, but only checking PHP 4 as that is what is running on the server. However, I had an email asking about PHP 5 , so now there is a PHP 5 version too 🙂

The syntax checker is a pretty simple layer over the PHP command line option “php -l” and also uses the PHP highlight_file function. The main complication is parsing the HTML outputs of both as they change between versions of PHP!

There is also a download archive so you can also have it running locally on your own system.

watch this space …[back]

What’s wrong with dynamic binding?

Posted on July 20, 2009 by alan

Dynamic scoping/binding of variables has a bad name, rather like GOTO and other remnants of the Bad Old Days before Structured Programming saved us all¹. But there are times when dynamic binding is useful and looking around it is very common in web scripting languages, event propagation, meta-level programming, and document styles.

So is it really so bad?

Continue reading →

Strangely also the days when major advances in substance seemed to be more important than minor advances in nomenclature[back]

a simple PHP record sorter class

Posted on July 3, 2009 by alan

Not for the first time I needed to sort arrays of arrays in PHP (structures like tiny DB tables). I have previously written little wrapper functions round usort, but decided this time to make a small class. It is a simple, but generic utility, so popping it up in case useful to anyone.

The rest of this post has moved to a permanent page at:

http://www.alandix.com/blog/code/sorter/

programming as it could be: part 1

Posted on February 22, 2009 by alan

Over a cup of tea in bed I was pondering the future of business data processing and also general programming. Many problems of power-computing like web programming or complex algorithmics, and also end-user programming seem to stem from assumptions embedded in the heart of what we consider a programming language, many of which effectively date from the days of punch cards.

Often the most innovative programming/scripting environments, Smalltalk, Hypercard, Mathematica, humble spreadsheets, even (for those with very long memories) Filetab, have broken these assumptions, as have whole classes of ‘non-standard’ declarative languages. More recently Yahoo! Pipes and Scratch have re-introduced more graphical and lego-block style programming to end-users (albeit in the case of Pipes slightly techie ones).


Yahoo! Pipes (from Wikipedia article)	Scratch programming using blocks

What would programming be like if it were more incremental, more focused on live data, less focused on the language and more on the development environment?

Two things have particularly brought this to mind.

First was the bootcamp team I organised at the Winter School on Interactive Technologies in Bangalore¹. At the bootcamp we were considering “content development through the keyhole”, inspired by a working group at the Mobile Design Dialog conference last April in Cambridge. The core issue was how one could enable near-end-use development in emerging markets where the dominant, or only, available computation is the mobile phone. The bootcamp designs focused on more media content development, but one the things we briefly discussed was full code development on a mobile screen (not so impossible, after all home computers used to be 40×25 chars!), and where literate programming might offer some solutions, not for its original aim of producing code readable by others², but instead to allow very succinct code that is readable by the author.

if ( << input invalid >> )
    << error handling code >>
else
    << update data >>

(example of simple literate programming)

The second is that I was doing a series of spreadsheets to produce some Fitts’ Law related modelling. I could have written the code in Java and run it to produce outputs, but the spreadsheets were more immediate, allowed me to get the answers I needed when I needed them, and didn’t separate the code from the outputs (there were few inputs just a number of variable parameters). However, complex spreadsheets get unmanageable quickly, notably because the only way to abstract is to drop into the level of complex spreadsheet formulae (not the most readable code!) or VB scripting. But when I have made spreadsheets that embody calculations, why can’t I ‘abstract’ them rather than writing fresh code?

I have entitled this blog ‘part 1’ as there is more to discuss than I can manage in one entry! However, I will return, and focus on each of the above in turn, but in particular questioning some of those assumptions embodied in current programming languages:

(a) code comes before data

(b) you need all the code in place before you can run it

(d) the programming language and environment are separate

In my PPIG keynote last September I noted how programming as an activity has changed, become more dynamic, more incremental, but probably also less disciplined. Through discussions with friends, I am also aware of some of the architectural and efficiency problems of web programming due to the opacity of code, and long standing worries about the dominance of limited models of objects³

So what would programming be like if it supported these practices, but in ways that used the power of the computer itself to help address some of the problems that arise when these practices address issues of substantial complexity?

And can we allow end-users to more easily move seamlessly from filling in a spreadsheet, to more complex scripting?

The winter school was part of the UK-India Network on Interactive Technologies for the End-User. See also my blog “From Anzere in the Alps to the Taj Bangelore in two weeks“[back]
such as Knuth‘s “TeX: the program” book consisting of the full source code for TeX presented using Knuth’s original literate programming system WEB.[back]
I have often referred to object-oriented programming as ‘western individualism embodied in code’.[back]

PPIG2008 and the twenty first century coder

Posted on September 16, 2008 by alan

Last week I was giving a keynote at the annual workshop PPIG2008 of the Psychology of Programming Interest Group. Before I went I was politely pronouncing this pee-pee-eye-gee … however, when I got there I found the accepted pronunciation was pee-pig … hence the logo!

My own keynote at PPIG2008 was “as we may code: the art (and craft) of computer programming in the 21st century” and was an exploration of the changes in coding from 1968 when Knuth published the first of his books on “the art of computer programming“. On the web site for the talk I’ve made a relatively unstructured list of some of the distinctions I’ve noticed between 20th and 21st Century coding (C20 vs. C21); and in my slides I have started to add some more structure. In general we have a move from more mathematical, analytic, problem solving approach, to something more akin to a search task, finding the right bits to fit together with a greater need for information management and social skills. Both this characterisation and the list are, of course, a gross simplification, but seem to capture some of the change of spirit. These changes suggest different cognitive issues to be explored and maybe different personality types involved – as one of the attendees, David Greathead, pointed out, rather like the judging vs. perceiving personality distinction in Myers-Briggs¹.

One interesting comment on this was from Marian Petre, who has studied many professional programmers. Her impression, and echoed by others, was that the heavy-hitters were the more experienced programmers who had adapted to newer styles of programming, whereas the younger programmers found it harder to adapt the other way when they hit difficult problems. Another attendee suggested that perhaps I was focused more on application coding and that system coding and system programmers were still operating in the C20 mode.

The social nature of modern coding came out in several papers about agile methods and pair programming. As well as being an important phenomena in its own right, pair programming gives a level of think-aloud ‘for free’, so maybe this will also cast light on individual coding.

Margaret-Anne Storey gave a fascinating keynote about the use of comments and annotations in code and again this picks up the social nature of code as she was studying open-source coding where comments are often for other people in the community, maybe explaining actions, or suggesting improvements. She reviewed a lot of material in the area and I was especially interested in one result that showed that novice programmers with small pieces of code found method comments more useful than class comments. Given my own frequent complaint that code is inadequately documented at the class or higher level, this appeared to disagree with my own impressions. However, in discussion it seemed that this was probably accounted for by differences in context: novice vs. expert programmers, small vs large code, internal comments vs. external documentation. One of the big problems I find is that the way different classes work together to produce effects is particularly poorly documented. Margaret-Anne described one system her group had worked on² that allowed you to write a tour of your code opening windows, highlighting sections, etc.

I sadly missed some of the presentations as I had to go to other meetings (the danger of a conference at your home site!), but I did get to some and was particularly fascinated by the more theoretical/philosophical session including one paper addressing the psychological origins of the notions of objects and another focused on (the dangers of) abstraction.

The latter, presented by Luke Church, critiqued Jeanette Wing‘s 2006 CACM paper on Computational Thinking. This is evidently a ‘big thing’ with loads of funding and hype … but one that I had entirely missed :-/ Basically the idea is to translate the ways that one thinks about computation to problems other than computers – nerds rule OK. The tenet’s of computational thinking seem to overlap a lot with management thinking and also reminded me of the way my own HCI community and also parts of the Design (with capital D) community in different ways are trying to say they we/they are the universal discipline … well if we don’t say it about our own discipline who will …the physicists have been getting away with it for years 😉

Luke (and his co-authors) argument is that abstraction can be dangerous (although of course it is also powerful). It would be interesting perhaps rather than Wing’s paper to look at this argument alongside Jeff Kramer’s 2007 CACM article “Is abstraction the key to computing?“, which I recall liking because it says computer scientists ought to know more mathematics 🙂 🙂

I also sadly missed some of Adrian Mackenzie‘s closing keynote … although this time not due to competing meetings but because I had been up since 4:30am reading a PhD thesis and after lunch on a Friday had begin to flag! However, this was no reflection an Adrian’s talk and the bits I heard were fascinating looking at the way bio-tech is using the language of software engineering. This sparked a debate relating back to the overuse of abstraction, especially in the case of the genome where interactions between parts are strong and so the software component analogy weak. It also reminded me of yet another relatively recent paper³ on the way computation can be seen in many phenomena and should not be construed solely as a science of computers.

As well as the academic content it was great to be with the PPIG crowd they are a small but very welcoming and accepting community – I don’t recall anything but constructive and friendly debate … and next year they have PPIG09 in Limerick – PPIG and Guiness what could be better!

David has done some really interesting work on the relationship between personality types and different kinds of programming tasks. I’ve seen him present before about debugging and unfortunately had to miss his talk at PPIG on comprehension. Given his work has has shown clearly that there are strong correlations between certain personality attributes and coding, it would be good to see more qualitative work investigating the nature of the differences. I’d like to know whether strategies change between personality types: for example, between systematic debugging and more insight-based scan and see it bug finding. [back]
but I can’t find on their website :-([back]
Perhaps 2006/2007 in either CACM or Computer Journal, if anyone knows the one I mean please remind me![back]