On Sun, 2 Apr 2006, Dan McGhee wrote:
> There are also "back ups" and actually creating a backup. In thinking about
> all of this I have generated the following questions:
>
> 1. What are the pros and cons of creating an actual back up with something
> like AMANDA or DAR vs copying the / directory tree to a removable media?
Respectfully, I think this is the wrong question. Start by considering
how you are going to *restore* the files. In part, that depends on what
hardware you have available. Consider both deleted/mis-edited files,
files that end up in lost+found, and bare-metal recovery.
Currently, I have a rescue CD based on the uclibc 'rootfs' tarball [ to
save time when creating it, and space when it runs ] which has a minimal
set of tools in an initrd, and some more tools in the body of the CD
which it mounts as /usr. I can use this, umount /usr, and still mount a
CD or DVD and read backups from it. I'm sure that this process is a lot
easier if you have the luxury of two CD/DVD drives!
More generally, my home and root filesystems are backed up over nfs to
a RAID1 mirror. From there I occasionally tar up the root filesystems
to a DVD (in general, systems I care about recovering don't change very
often). The home data on workstations is not important enough to back
up beyond the mirror (config files like ~/.xinitrc are copied manually
to the server when they change).
My main /home data is on the server, that has similar backups, but
also from time to time I write it to tape (a fully manual process).
You may also want to consider space requirements - to restore
individual files, you may need to read a whole backup to a temporary
location. In my case, my tape backups are tarballs, currently about 25
GB for /home.
> 2. What are the pros and cons of creating archives vs. copying files and
> directories?
Space. But, with an accessible backup of files, it is much easier to
restore an individual file that got deleted or changed in error.
> 3. What are the pros and cons of using tar vs. using cpio?
Most people here are probably more familiar with tar and therefore less
liable to make mistakes. I believe gnu tar can do everything, unlike
older versions of tar.
> 4. Whether straight files or archives are there many risks in compression.
> If so, do the space saving advantages on removable media outweigh the risks?
>
Compression (bzip2, gzip) is not lossy. If your RAM becomes defective,
reading a file into it will cause corruption, irrespective of whether
you then compress it, but this is more likely to be noticed when bzip2
detects an error in decompressing the file. The downside to compression
is the cpu overhead to compress and decompress, particularly with bzip2,
and therefore the extra time.
> Space will probably not be an issue. Once I get it configured and running I
> have dual layer DVD capability.
If you have infinite space, and the media is sufficiently fast, then
don't bother compressing. Otherwise, test what is best for you - e.g.
will compression allow you to use faster (and more compatible)
single-layer media ?
More generally, the problems with backups include: not making them, not
testing them, running out of time or space while trying to create them,
not being able to restore them because the specific hardware died, and
backups which become unreadable over time. This usually means that once
you have a working procedure, there is a major cost (not just monetary)
in changing it.
Ken
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