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

[ken <gebser@xxxxxxxxxxxx>]

On 03/16/2012 02:53 PM Les Mikesell wrote:
> On Fri, Mar 16, 2012 at 1:19 PM, ken <gebser@xxxxxxxxxxxx> 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.

Thanks, Les!  That fleshes the process out a little more for me.  But 
what does it mean that a snapshot is "active"?


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