[CentOS] CentOS Server Backup Options: at runlevel 5? ... w/snapshot

[Les Mikesell]

On Fri, Mar 16, 2012 at 1:19 PM, ken <gebser at mousecar.com> wrote:
>
>>>
>> I find that LVM snapshots are useful to insure data integrity.  for
>> example, I backup my mysql databases by stopping the mysql server,
>> taking an LVM snapshot and restarting it.  The whole snapshot process
>> probably takes less then 15 seconds.  Then I backup the snapshot LVM and
>> it doesn't matter how long it takes.  You must make sure that your
>> snapshot volume is large enough that you won't run out of space before
>> deleting the snapshot.
>
> Thanks for the tip.  The details of how a snapshot works has long evaded
> me.  For example, when files are open but unsaved, what is written to
> the snapshot...?  And is the entire contents of a file recorded by the
> snapshot... or something else?

LVM snapshots just present a view of the disk as it was when the
snapshot was taken.  To do that, whenever you write to the disk with a
snapshot active, it first copies the old data into the snapshot.
Blocks that don't change don't need to be copied.    This doesn't do
anything by itself to make the files consistent, but you might stop
your apps momentarily, make the snapshot, then let them run again
while the snapshot is being backed up.

-- 
  Les Mikesell
    lesmikesell at gmail.com