Snapshotting for rollback after updates was[ Re: Drawing lessons from fatal SELinux bug #1054350]

[Chris Murphy]

On Jan 26, 2014, at 11:38 AM, Simo Sorce <simo at redhat.com> wrote:

> On Sat, 2014-01-25 at 17:54 -0700, Chris Murphy wrote:
>> On Jan 25, 2014, at 4:12 PM, Adam Williamson <awilliam at redhat.com> wrote:
>>> 
>>> * Do an offline update that includes Foo v2.0
>>> * Boot the updated system, run Foo, it migrates its configuration to
>>> some new scheme
>>> * Realize there was something wrong with the update, roll it back
>>> * Run Foo again, find it doesn't work because it's been migrated to the
>>> new config scheme which the old version of Foo doesn't work with
>> 
>> I would grumble, but a configuration file being updated and made
>> incompatible with the prior version would be tolerated. Ideally the
>> application makes an unmodified copy. If it doesn't, new school
>> restore with --reflink from snapshot, regular cp if using LVM thinp
>> snapshots, and old school just restore the file from a conventional
>> backup. Not such a big deal.
>> 
>> If it's something far less throw away than configuration files being
>> changed, it's a bit more complicated how badly and quickly the
>> conversation degrades. But I can hardly recall a recent example of
>> this happening. It's just not that common in my experience.
> 
> What about mail application change the format of the mail folders ?

Good question because I experienced this recently. So the way Apple does this on OS X with Mail, there is no such thing as a mail format change during the life of a major OS version. So major OS versions are 10.7, 10.8, and now 10.9. So it's impossible the mail format would change between 10.7.1 and 10.7.2 in such a way that 10.7.2 mail could not be read by the 10.7.1 or 10.7.0 mail application. I can upgrade and downgrade all along 10.7.x and the file format is the same.

Recently I learned that there are two mail formats. There's the every day used format that is apparently completely incompatible between major versions of Mail. That is, 10.8.x Mail will not read (or write) 10.7.x Mail. I must first do an export from 10.7 Mail, to an intermediate format, that 10.8 Mail can read. Likewise, I can export 10.8 Mail in this format, and 10.7 Mail can also read it. And even this pissed me off as well as several thousand users (at least) based on Apple's community forums on the subject because most of us expect to be able to directly import 10.7 mail into 10.8 Mail. But at least there is a means to downgrade even to a major version of Mail should I wish.

> It happens, and it is *not* data you want to risk throwing away. There
> are many other examples like this especially on the server side.

Well, the mail servers regularly get updated by the company I pay for such things, and I've never noticed the change. It uses IMAP so I don't think the server even cares, its just a bunch of folders and files. Apple Mail and the Android phone read mails from the same server, files and folders without problems. So from my perspective nothing has changed format wise.


Chris Murphy