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

John R Pierce pierce at hogranch.com
Fri Mar 16 19:55:31 EDT 2012


On 03/16/12 4:10 PM, ken wrote:
> Thanks, Les!  That fleshes the process out a little more for me.  But
> what does it mean that a snapshot is "active"?

means one exists.    LVM maps logical volumes (LV) onto a set of PP 
(physical partitions, also called extents) that are maybe 4-16MB each 
(this is configurable when you create the partition).    when you create 
a snapshot, nothing actually happens until the file system is written, 
the snapshot is just another view of the PPs assembled into a new LV 
thats pointing to the same blocks...  when the original LV is written, 
the existing data is copied to new PP's grabbed from the free pool, and 
the snapshot is pointed at these saved copies, then the file system is 
allowed to write onto the original PPs.    when you delete the snapshot 
these new PPs containing the original data are lost.   if you revert the 
file system to the snapshot, then the PPs that were updated are deleted 
and the snapshot becomes the LV.

the implementation of this in Linux came from IBM's AIX Unix, where the 
Logical Volume Manager is quite nicely integrated with their JVM/JVM2 
file systems (grow the file system and the LV's are expanded automatically).

-- 
john r pierce                            N 37, W 122
santa cruz ca                         mid-left coast



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