On 03/16/2012 02:53 PM Les Mikesell wrote: > 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. Thanks, Les! That fleshes the process out a little more for me. But what does it mean that a snapshot is "active"?