[Discuss] Backing up LVM partitions using snapshots

[gaf at blu.org (Jerry Feldman)]

On 12/12/2011 11:06 AM, Mark Woodward wrote:
> In your example, a duplicate reducing backup would ignore most of the changes. 
>
>
> Edward Ned Harvey <blu at nedharvey.com> wrote:
>
>>> From: markw at mohawksoft.com [mailto:markw at mohawksoft.com]
>>> Sent: Sunday, December 11, 2011 2:48 PM
>>>
>>> I will argue that an rsync will NEVER be more effective unless you
>>> actively wipe the blocks where a file once existed. 
>> for (( i=0 ; i<200 ; i++ )) ; do
>> mkdir temp
>> cp datafile temp
>> run_test $i >> testresults.txt
>> rm -rf temp
>> done
>>
>> In this case, rsync is what you want, because it ignores files that don't
>> exist.  But a block level backup will backup all the blocks that were ever
>> contained in any of the (now removed) copies of the datafile.
>>
>> I don't know what users you support, but I support engineers who run this
>> type of test all the time.  They create test work dirs, they perform
>> volatile work in there, store the results of the test, and remove their
>> scratch dir.
>>
>> The block level backup you're talking about is great, under the assumption
>> that you basically just add data to a filesystem.  It's terrible when you
>> add & remove data from the filesystem.  I stand by my claim:  Important to
>> know if it's suitable for your purposes, whoever you are, the consumer who
>> might consider using this.
>>
I have always preferred a file-oriented backup approach, but I have also
been burned. I used to build tarballs until my backup of my home
directory placed a VM in the tarball on a 32-bit Linux, and the drive
where my home drive was crashed. I was able to restore everything up to
the VM that was larger than 3GB. Eventually, I paid to extract the data
from the hard drive because the I lost my email archives and checkbook.

With today's larger HDs and/or inexpensive NAS systems, like the WD
MyBook, you can use rsync's --link-dest so you can have the equivalent
of both a full backup and an incremental backup. At work, I use
rsnapshot to back up our systems and keep about 20 days online, but New
York backs us up daily and performs a tape backup so in the case of a
disaster that would knock out our systems, we would lose minimal data.
The upside of this scheme is that the most frequent need for a backup is
that someone accidentally deletes a file.

At home, I do a similar thing except I don't do a remote backup (I know
very well I should :-).

The disadvantage of a file-oriented backup is if you have to do a full
system restore, but this is rather rare. At home, if I were to lose my
primary HDs I would essentially have to spend time to rebuild the entire
system.

-- 
Jerry Feldman <gaf at blu.org>
Boston Linux and Unix
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