This is losely related to:
Subject: Re: [PLUG] Fixiing Slackware upgrade mistake

No idea about Slackware ...

Old kernel versions through GRUB:
---------------------------------
Bunch of distributions (openSuSE, Ubuntu, Fedora) can boot out of
previous kernel version as long as you do not remove/uninstall them.
This is done automagically by creating Grub entries for them when
installing new kernel by the install script embedded in .rpm or .deb.

Btrfs disk/volume snapshots in openSuSE:
----------------------------------------
As far as I know - openSuSE is the only distro out of the box
supporting restore of any previous OS-partition state (not just kernel
changes) stored as file system snapshots. Snapshots are enabled by
installing the OS on btrfs partition (default). They are automagically
created every time you install something, run update, change config
using it Yast2 (SuSE config tool) or manually on-demand using snapper
tool. By their nature, snapshots store full disk history and take
almost no time to create.

So, as long as you have up to date disk snapshot you can make config
changes safely. Snapper can be also used to delete old snapshots or
show differences/file changes between them, etc.

Naturally, snapshots cost disk space. They are diferential, so they
store only the disk changes made between them. Despite that, small file
system changes can add up over couple of hunderds of them over time.
Ocasional clean up is good idea, though you can configure snapper to
only keep some number of them - there is sensible default behavior - I
do not have to do anything to recycle them by default.

Since the snapshots are cheap and can be created for volumes or sub-
volumes (sort of partitions inside Btrfs partition) - one can setup
data/home partition snapshot creation & rotation by cron every
hour/day/week/month/etc. It is pretty handy.

Note of caution if you are not experienced with CoW filesystems such as
Btrfs or Zfs - it makes sense to turn off CoW for volumes/sub-volumes
storing files with a lot of frequent changes such as VM images, swap
files or database files. Alternatively, one can keep such files on Ext4
or Xfs partition.

-Tomas
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