rootfs snapshots and rollback (i.e. testing updates)

rootfs snapshots and rollback (i.e. testing updates)

[Tomasz Chmielewski]

Is it possible, with current btrfs:

- to take a rootfs snapshot (i.e. prior to a major update),

- do changes in the root filesystem (i.e. install major update),

- if we don't like what the major update did to the system (rootfs), 
"rollback" the snapshot and make it the "original" rootfs again 
(perhaps, with a reboot in between).



If it's possible, what would be the steps/commands?


-- 
Tomasz Chmielewski
http://wpkg.org

Re: rootfs snapshots and rollback (i.e. testing updates)

[John Dong]

Hi,

I asked this question about a month ago, and the answer is roughly  
"It's theoretically possible with minor feature implementations to  
btrfs, though nobody's done it yet"
On Nov 9, 2009, at 10:00 AM, Tomasz Chmielewski wrote:

> Is it possible, with current btrfs:
>
> - to take a rootfs snapshot (i.e. prior to a major update),
>
> - do changes in the root filesystem (i.e. install major update),
>
> - if we don't like what the major update did to the system (rootfs),  
> "rollback" the snapshot and make it the "original" rootfs again  
> (perhaps, with a reboot in between).
>
>
>
> If it's possible, what would be the steps/commands?
>
>
> -- 
> Tomasz Chmielewski
> http://wpkg.org
> --
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Re: rootfs snapshots and rollback (i.e. testing updates)

[Chris Ball]

Hi,

   > Is it possible, with current btrfs:

Yes, I think so.

   > - to take a rootfs snapshot (i.e. prior to a major update),

btrfsctl -s newsnap /

   > - do changes in the root filesystem (i.e. install major update),
   > 
   > - if we don't like what the major update did to the system
   > (rootfs), "rollback" the snapshot and make it the "original"
   > rootfs again (perhaps, with a reboot in between).

Before rebooting, edit whatever mounts your root partition (initrd,
fstab, kernel argument) to add a "subvol=newsnap" mount argument.

An obvious way to make this nicer would be to:
   * have the package manager create the snapshot before modifying the
     system, with a timestamp.
   * modify the bootloader to give a choice of snapshots at boot-time.

Note that you're rolling back *all* rootfs changes, not merely the
changes that the package manager made, so it wouldn't be correct to
think of this as a way to only rollback package manager transactions.

- Chris.
-- 
Chris Ball   <cjb@laptop.org>
One Laptop Per Child

Re: rootfs snapshots and rollback (i.e. testing updates)

[Tomasz Chmielewski]

Chris Ball wrote:
> Hi,
> 
>    > Is it possible, with current btrfs:
> 
> Yes, I think so.
> 
>    > - to take a rootfs snapshot (i.e. prior to a major update),
> 
> btrfsctl -s newsnap /
> 
>    > - do changes in the root filesystem (i.e. install major update),
>    > 
>    > - if we don't like what the major update did to the system
>    > (rootfs), "rollback" the snapshot and make it the "original"
>    > rootfs again (perhaps, with a reboot in between).
> 
> Before rebooting, edit whatever mounts your root partition (initrd,
> fstab, kernel argument) to add a "subvol=newsnap" mount argument.

So, if I understand it correctly, it's not really "rolling back".

Rather, with a "failed upgrade", we would mount a "newsnap" snapshot to 
use the old rootfs again.


What's still left here would be:

- remove the "failed upgrade" "(sub)volume" (which is "/" now?)

- turn everything to such a state, so that you can mount the 
old/original rootfs without adding any "subvol=newsnap" mount arguments


Correct me if I'm wrong.


-- 
Tomasz Chmielewski
http://wpkg.org

Re: rootfs snapshots and rollback (i.e. testing updates)

[John Dong]

On Nov 10, 2009, at 4:53 AM, Tomasz Chmielewski wrote:

> Chris Ball wrote:
>> Hi,
>>   > Is it possible, with current btrfs:
>> Yes, I think so.
>>   > - to take a rootfs snapshot (i.e. prior to a major update),
>> btrfsctl -s newsnap /
>>   > - do changes in the root filesystem (i.e. install major update),
>>   >    > - if we don't like what the major update did to the system
>>   > (rootfs), "rollback" the snapshot and make it the "original"
>>   > rootfs again (perhaps, with a reboot in between).
>> Before rebooting, edit whatever mounts your root partition (initrd,
>> fstab, kernel argument) to add a "subvol=newsnap" mount argument.
> 
> So, if I understand it correctly, it's not really "rolling back".
> 
> Rather, with a "failed upgrade", we would mount a "newsnap" snapshot to use the old rootfs again.
> 
> 
> What's still left here would be:
> 
> - remove the "failed upgrade" "(sub)volume" (which is "/" now?)
> 
> - turn everything to such a state, so that you can mount the old/original rootfs without adding any "subvol=newsnap" mount arguments
> 
> 
> Correct me if I'm wrong.

You're right. What's needed is a way to set the new default subvol